Method and apparatus for contact image sensing

ABSTRACT

A contact image sensor having an illumination source; a first SBG array device; a transmission grating; a second SBG array device; a waveguiding layer including a multiplicity of waveguide cores separated by cladding material; an upper clad layer; and a platen. The sensor further includes: an input element for coupling light from the illumination source into the first SBG array; a coupling element for coupling light out of the cores into output optical paths coupled to a detector having at least one photosensitive element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. Ser. No. 14/910,921 filedFeb. 8, 2016, now U.S. Pat. No. 9,727,772 which is the U.S. nationalphase of PCT Application No. PCT/GB2014/000295 filed on Jul. 30, 2014,which claims the benefit of U.S. Provisional Patent Application No.61/958,552 filed on Jul. 31, 2013, the disclosures of which areincorporated in their entirety by reference herein.

The following patent applications are hereby incorporated by referenceherein in their entireties: PCT Application No.: PCT/GB2012/000680entitled IMPROVEMENTS TO HOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTALMATERIALS AND DEVICES with filing date 7 Jan. 2013; PCT Application No.:PCT/GB2012/000729 entitled METHOD AND APPARATUS FOR SWITCHING ELECTROOPTICAL ARRAYS with filing date 6 Sep. 2012; PCT Application No.:PCT/GB2013/000005 entitled CONTACT IMAGE SENSOR USING SWITCHABLE BRAGGGRATINGS with filing date 7 Jan. 2013; and U.S. Pat. No.: 8,354,640entitled OPTICALLY-BASED PLANAR SCANNER with issue date 15 Jan. 2013.

TECHNICAL FIELD

The present invention relates to an imaging sensor, and moreparticularly to a contact image sensor using electrically switchableBragg gratings.

BACKGROUND

A contact image sensor is an integrated module that comprises anillumination system, an optical imaging system and a light-sensingsystem - all within a single compact component. The object to be imagedis place in contact with a transparent outer surface (or platen) of thesensor. Well known applications of contact image sensors includedocument scanners, bar code readers and optical identificationtechnology. Another field of application is in biometric sensors, wherethere is growing interest in automatic finger print detection.Fingerprints are a unique marker for a person, even an identical twin,allowing trained personnel or software to detect differences betweenindividuals. Fingerprinting using the traditional method of inking afinger and applying the inked finger to paper can be extremelytime-consuming. Digital technology has advanced the art offingerprinting by allowing images to be scanned and the image digitizedand recorded in a manner that can be searched by computer. Problems canarise due to the quality of inked images. For example, applying too muchor too little ink may result in blurred or vague images. Further, theprocess of scanning an inked image can be time-consuming. A betterapproach is to use “live scanning” in which the fingerprint is scanneddirectly from the subject's finger. More specifically, live scans arethose procedures which capture fingerprint ridge detail in a mannerwhich allows for the immediate processing of the fingerprint image witha computer. Examples of such fingerprinting systems are disclosed inFishbine et al. (U.S. Pat. Nos. 4,811,414 and 4,933,976); Becker (U.S.Pat. No. 3,482,498); McMahon (U.S. Pat. No. 3,975,711); and Schiller(U.S. Pat. Nos. 4,544,267 and 4,322,163). A live scanner must be able tocapture an image at a resolution of 500 dots per inch (dpi) or greaterand have generally uniform gray shading across a platen scanning area.There is relevant prior art in the field of optical data processing inwhich optical waveguides and electro-optical switches are used toprovide scanned illumination. One prior art waveguide illuminator isdisclosed in U.S. Pat. No. 4,765,703. This device is an electro-opticbeam deflector for deflecting a light beam within a predetermined rangeof angle. It includes an array of channel waveguides and plural pairs ofsurface electrodes formed on the surface of a planar substrate of anelectro-optic material such as single crystal Lithium Niobate (LiNbO₃).

While the fingerprinting systems disclosed in the foregoing patents arecapable of providing optical or optical and mechanical fingerprintimages, such systems are only suitable for use at a central locationsuch as a police station. Such a system is clearly not ideal for lawenforcement and security applications where there is the need to performan immediate identity and background check on an individual while in thefield. In general, current contact image sensor technology tends to bebulky, low in resolution and unsuitable for operation in the field. Thusthere exists a need for a portable, high resolution, lightweight opticalcontact sensor for generating images in the field.

SUMMARY

It is an object of the present invention to provide a portable, highresolution, lightweight contact image sensor for generating images inthe field.

In a first embodiment of the invention a contact image sensor accordingto the principles of the invention comprises the following paralleloptical layers configured as a stack: an illumination means forproviding a collimated beam of first polarisation light; a first SBGarray device further comprising first and second transparent substratessandwiching an array of selectively switchable SBG column elements, andITO electrodes applied to opposing faces of the substrates and the SBGsubstrates together providing a first TIR light guide for transmittinglight in a first TIR beam direction; an air gap; a transmission grating;a third transparent substrate (low index glue layer); a SBG cover glass;a ITO layer; a second SBG array device comprising an array ofselectively switchable SBG column elements; a ITO layer; a barrier film;a waveguiding layer comprising a multiplicity of waveguide coresseparated by cladding material having a generally lower refractive indexthan the cores, the cores being disposed parallel to the first beamdirection; an upper clad layer having a generally lower refractive indexthan the cores; a priming layer; and a platen. The apparatus furthercomprises: means for coupling light from the illumination means into thefirst TIR light guide; means for coupling light out of the core into anoutput optical path; and a detector comprising at least onephotosensitive element, the photosensitive element being opticallycoupled to at least one the core. ITO electrodes are applied to theopposing faces of the third transparent substrate and the waveguidinglayer. The column elements of the first and second SBG arrays havelonger dimensions disposed orthogonally to the first TIR beam direction.In one embodiment of the invention the air gap may be replaced by arefracting material layer.

Each SBG element in the first and second SBG arrays has a diffractingstate when no electric field is present across the ITO electrodes and anon-diffracting state when an electric field is present across the ITOelectrodes, the SBG elements diffracting only the first polarizationlight.

The elements of the second SBG device which are in a non-diffractingstate have a generally lower refractive index than the cores. The thirdtransparent substrate has a generally lower refractive index than thecores. At any time one element of the first SBG array is in adiffracting state, one element of the second SBG array is in adiffracting state, and all other elements of the first and second are ina non-diffracting state.

In one embodiment of the invention an active SBG element of the firstSBG array in a diffracting state diffracts incident first TIR lightupwards into a first beam direction. The transmission grating diffractsthe first beam direction light upwards into a second beam direction.When contact is made with an external material at a point on the platena portion of the second beam direction light incident at the point onthe platen contacted by said external material is transmitted out of theplaten. All other light incident on the outer surface of the platen isreflected downwards in a third optical path, the third optical pathtraversing the cores. An active SBG element of the second SBG arrayalong the third beam direction diffracts the third angle light downwardsinto a fourth beam direction. The fourth beam direction light isreflected upwards at the third transparent substrate into a fifth beamdirection. The fifth beam direction light exceeds the critical angle setby the core/clad interface and the critical angle set by one of thecore/second SBG array or second SBG array/third transparent substrateinterfaces, providing a TIR path to the detector. The first to fifthbeam directions lie in a plane orthogonal to the first SBG array.

In one embodiment of the invention the third transparent substrate has agenerally lower refractive index than the element of the second SBGarray in its diffracting state.

In one embodiment of the invention the third transparent substrate has agenerally lower refractive index than the element of the second SBGarray in its non-diffracting state.

In one embodiment of the invention the apparatus further comprises atransparent slab of index lower than that of the third substratedisposed between the third substrate and the transmission grating.

In one embodiment of the invention the output from detector arrayelement is read out in synchronism with the switching of the elements ofthe first SBG array.

In one embodiment of the invention the apparatus further comprises atransparent slab of index lower than that of the third substratedisposed between the third substrate and the transmission grating. Anactive SBG element of the first SBG array in a diffracting statediffracts incident first TIR light upwards into a first optical path ina plane orthogonal to the first SBG array. The transmission gratingdiffracts the first optical path light upwards into a second opticalpath. When contact is made with an external material at a point on theplaten a portion of the second beam direction light incident at thepoint on the platen contacted by said external material is transmittedout of the platen. All other light incident on the outer surface of theplaten is reflected downwards in a third optical path, the third opticalpath traversing the cores. The third optical path traverses the core. Anactive SBG element of the second SBG array along the third optical pathdiffracts the third angle light downwards into a fourth optical path.The fourth optical path light is reflected upwards at least one of thethird transparent substrate or the slab into a fifth optical path. Thefifth optical path light exceeds the critical angle set by the core/cladinterface and the critical angle set by one of the core/second SBGarray, second SBG array/third substrate or third substrate/slabinterfaces, providing a TIR path to the detector. The first to fifthoptical paths lie in a plane orthogonal to the first SBG array.

In one embodiment of the invention the illumination means comprises alaser and a collimator lens.

In one embodiment of the invention the means for coupling light from theillumination means into the first TIR light guide is a grating.

In one embodiment of the invention the means for coupling light from theillumination means into the first TIR light guide is a prismaticelement.

In one embodiment of the invention the means for coupling the second TIRlight into the waveguide is a grating.

In one embodiment of the invention the means for coupling light out ofthe waveguide is a grating.

In one embodiment of the invention the first and second SBG arrays eachcomprise continuous SBG layers and the selectively switchable elementsof first and second SBG arrays are defined by configuring at least oneof the transparent electrodes as a multiplicity of selectivelyswitchable electrode elements.

In one embodiment of the invention an air gap is provided between thefirst SBG array and the transmission grating.

In one embodiment of the invention the sensor further comprises apriming layer between the upper clad layer and the platen.

In one embodiment of the invention at least one of the transparentelectrodes and substrates sandwiches a barrier layer.

In one embodiment of the invention the transparent substrates arefabricated from plastic.

In one embodiment of the invention the transparent substrates arefabricated from a polycarbonate

In one embodiment of the invention the waveguide cores are fabricatedfrom an electrically conductive material.

In one embodiment of the invention the waveguide cores are fabricatedfrom PDOT

In one embodiment of the invention the waveguide cores are fabricatedfrom CNT.

In one embodiment of the invention the waveguides are fabricated fromCNT using a lift-off stamping process.

In one embodiment of the invention the waveguides are coupled to lineararray of detectors.

In one embodiment of the invention the waveguides are coupled to a twodimensional detector array.

In one embodiment of the invention the transparent electrodes arefabricated from ITO.

In one embodiment of the invention the transparent electrodes arefabricated from CNT.

In one embodiment of the invention the transparent electrodes arefabricated from PDOT.

In one embodiment of the invention the waveguides are fabricated fromPDOT.

In one embodiment of the invention the waveguide cores are fabricatedfrom a conductive photopolymer the waveguide cores and second SBG arrayelements being disposed such that only the portions off the SBG arrayelements lying directly under the waveguide cores are switched.

In one embodiment of the invention the SBG arrays are fabricated using areverse mode HPDLC.

In one embodiment of the invention there is provided a method of makinga contact image measurement comprising the steps of:

-   a) providing an apparatus comprising the following parallel optical    layers configured as a stack: an illumination means for providing a    collimated beam of first polarisation light; a first SBG array    device further comprising first and second transparent substrates    sandwiching an array of selectively switchable SBG column elements,    and ITO electrodes applied to opposing faces of the substrates and    the SBG substrates together providing a first TIR light guide for    transmitting light in a first beam direction; an air gap; a    transmission grating; a transparent substrate (low index glue); an    SBG cover glass; a ITO layer; a second SBG array device comprising    array of selectively switchable SBG column elements; a ITO layer; a    barrier film; a waveguiding layer comprising a multiplicity of    waveguide cores separated by cladding material having a generally    lower refractive index than the cores, the cores being disposed    parallel to the first beam direction; an upper clad layer having a    generally lower refractive index than the cores(which is also    referred to as the bottom buffer); a priming layer; a platen; and    further comprising: means for coupling light from the illumination    means into the first TIR light guide; means for coupling light out    of the waveguide into an output optical path; and a detector    comprising at least one photosensitive element, wherein ITO    electrodes are applied to the opposing faces of the substrate and    the waveguide core;-   b) an external material contacting a point on the external surface    of the platen;-   c) sequentially switching elements of the first SBG array into a    diffracting state, all other elements being in their non-diffracting    states;-   d) sequentially switching elements of the second SBG array into a    diffracting state, all other elements being in their non-diffracting    states;-   e) each diffracting SBG element of the first SBG array diffracting    incident first TIR light upwards into a first optical path,-   f) the transmission grating diffracting the first optical path light    upwards into a second optical path,-   g) a portion of the second optical path light incident at the point    on the platen contacted by said external material being transmitted    out of the platen and any other light being reflected downwards in a    third optical path, the third optical path traversing one the core,-   h) an active SBG element of the second SBG array along the third    optical path diffracting the third angle light downwards into a    fourth optical path,-   i) the fourth optical path light being reflected upwards into a    fifth optical path at the third substrate, the fifth optical path    light exceeding the critical angle set by the core/clad interface    and the critical angle set by one of the core/second SBG array or    second SBG array/third substrate interfaces, and proceeding along a    TIR path to the detector.

The first to fifth optical paths lie in a plane orthogonal to the firstSBG array.

In one embodiment of the invention the method further comprises atransparent slab of index lower than the substrate disposed between thesubstrate and the transmission grating, such that the fourth opticalpath light is reflected upwards at the substrate into a fifth opticalpath and the fifth optical path light exceeds the critical angle set bythe core/clad interface and the critical angle set by one of thecore/second SBG array, second SBG array/third substrate or thirdsubstrate/slab interfaces, providing a TIR path to the detector.

In one embodiment of the invention the air gap may be replaced by arefracting material layer.

In one embodiment of the invention the illumination means comprises amultiplicity of laser illumination channels, each said channelcomprising a laser and collimating lens system. The illumination meansprovides a multiplicity of collimated, abutting beams of rectangularcross section.

In one embodiment of the invention the illumination means comprises alaser and a collimator lens. The said illumination means provides acollimated beam of rectangular cross section.

In one embodiment of the invention the optical wave guiding structurecomprises a multiplicity of parallel strip cores separated by claddingmaterial.

In one embodiment of the invention the optical wave guiding structurecomprises a single layer core.

In one embodiment of the invention the SBG elements are strips alignednormal to the propagation direction of the TIR light.

In one embodiment of the invention the SBG elements are switchedsequentially across the SBG array and only one SBG element is in itsdiffracting state at any time.

In one embodiment of the invention the sensor further comprises a microlens array disposed between the SBG device and the first cladding layer.

In one embodiment of the invention the means for coupling light from theillumination means into the first TIR light guide is a grating.

The illumination device of claim the means for coupling light from theillumination means into the first TIR light guide is a prismaticelement.

In one embodiment of the invention the means for coupling the second TIRlight into the wave-guiding structure is a grating.

In one embodiment of the invention the means for coupling light out ofthe wave-guiding structure is a grating.

In one embodiment of the invention, the output light from the waveguiding device is coupled into a linear detector array.

In one embodiment of the invention, the output light from the waveguiding device is coupled into a two dimensional detector array.

In one embodiment of the invention a contact image sensor furthercomprises a half wave retarder array disposed between the air gap andthe transmission grating. The half wave retarder array comprises anarray of column-shaped elements sandwiched by transparent substrates.Each retarder element in the half wave retarder array is switchablebetween a polarization rotating state in which it rotates thepolarization of incident light through ninety degrees and a nonpolarization rotating state. The column elements of the half waveretarder array have longer dimensions disposed parallel the first TIRbeam direction. Each half wave retarder array element overlaps at leastone strip element of the first SBG array. At any time one element of thefirst SBG array is in a diffracting state and is overlapped by anelement of the half wave retarder array in its non-polarization rotatingstate, one element of the second SBG array is in a diffracting state,all other elements of the first and second SBG arrays are in anon-diffracting state and all other elements of the half wave retarderarray are in their polarization rotating states.

One embodiment of the invention uses a SBG waveguiding structure. Inthis embodiment there is provided a contact image sensor comprising thefollowing parallel optical layers configured as a stack: an illuminationmeans for providing a collimated beam of first polarisation light; afirst SBG array device further comprising first and second transparentsubstrates sandwiching an array of selectively switchable SBG column,and transparent electrodes applied to opposing faces of said substrate,the SBG substrates together providing a first TIR light guide fortransmitting light in a first TIR beam direction; a transmissiongrating; a second SBG array device further comprising third and fourthtransparent substrates sandwiching a multiplicity of high index HPDLCregions separated by low index HPDLC regions and patterned transparentelectrodes applied to opposing faces of the substrates; and a platen.The apparatus and further comprises: means for coupling light from theillumination means into the first TIR light guide; means for couplinglight out of the second SBG array device into an output optical path;and a detector comprising at least one photosensitive element. The highindex regions provide waveguiding cores disposed parallel to the firstbeam direction. The low index HPDLC regions provide waveguide cladding.The third and fourth substrate layers have a generally lower refractiveindex than the cores. The patterned electrodes applied to the thirdsubstrate comprise column shaped elements defining a multiplicity ofselectively switchable columns of SBG elements which are alignedorthogonally to the waveguiding cores. The patterned electrodes appliedto the fourth substrate comprise elongate elements overlapping the lowindex HPDLC regions. The detector comprises an array of photosensitiveelements, each photosensitive element being optically coupled to atleast one waveguiding core. Each SBG element in the first and second SBGarrays is switchable between a diffracting state and a non-diffractingstate with the SBG elements diffracting only first polarization light.

In one embodiment of the invention based on an SBG waveguiding structurethe diffracting state exists when an electric field is applied acrossthe SBG element and a non diffracting state exists when no electricfield is applied.

In one embodiment of the invention based on an SBG waveguiding structurethe diffracting state exists when no electric field is applied acrossthe SBG element and the non diffracting states exists when an electricfield is applied.

In one embodiment based on an SBG waveguiding structure, at any time,one element of the first SBG array is in a diffracting state, oneelement of the second SBG array is in a diffracting state, and all otherelements of the first and second are in a non-diffracting state.

In one embodiment of the invention based on an SBG waveguiding structurean active SBG element of the first SBG array in a diffracting statediffracts incident first TIR light upwards into a first beam direction.The transmission grating diffracts the first beam direction lightupwards into a second beam direction. When contact is made with anexternal material at a point on the platen a portion of the second beamdirection light incident at the point on the platen contacted by theexternal material is transmitted out of the platen. Light incident onthe outer surface of the platen in the absence of external material isreflected downwards in a third optical path which traverses the cores.An active column of the second SBG array along the third beam directiondiffracts the third angle light into a second TIR path down thetraversed core towards the detector. The first to third optical pathsand the first and second TIR paths lie in a common plane.

In one embodiment of the invention based on an SBG waveguiding structurethe output from detector array element is read out in synchronism withthe switching of the elements of the first SBG array.

In one embodiment of the invention based on an SBG waveguiding structurethere is provided an air gap between the first SBG array and thetransmission grating.

In one embodiment of the invention based on an SBG waveguiding structurethere is provided a method of making a contact image measurementcomprising the steps of:

-   a) providing an apparatus comprising the following parallel optical    layers configured as a stack: an illumination means for providing a    collimated beam of first polarisation light; a first SBG array    device further comprising first and second transparent substrates    sandwiching an array of selectively switchable SBG column elements,    and transparent electrodes applied to opposing faces of the    substrates and the SBG substrates together providing a first TIR    light guide for transmitting light in a first beam direction; a    transmission grating; a transparent substrate; a second SBG array    device further comprising third and fourth substrates sandwiching a    multiplicity of high index HPDLC regions separated by low index    HPDLC regions and patterned transparent electrodes applied to    opposing faces of the substrates; a platen; and a detector; and    further comprising: means for coupling light from the illumination    means into the first TIR light guide; means for coupling light out    of the second SBG array device into an output optical path; and a    detector comprising at least one photosensitive element; the high    index regions providing waveguiding cores disposed parallel to the    first beam direction and the low index HPDLC regions providing    waveguide cladding; the substrates layers having a generally lower    refractive index than the cores, the patterned electrodes applied to    the third substrate defining a multiplicity of selectively    switchable columns orthogonal to the waveguiding cores and the    patterned electrodes applied to the fourth substrate overlapping the    low index HPDLC regions;-   b) an external material contacting a point on the external surface    of the platen;-   c) sequentially switching elements of the first SBG array into a    diffracting state, all other elements being in their non-diffracting    states;-   d) sequentially switching columns of the second SBG array device    into a diffracting state, all other columns being in their    non-diffracting states;-   e) each diffracting SBG element of the first SBG array diffracting    incident first TIR light upwards into a first optical path,-   f) the transmission grating diffracting the first optical path light    upwards into a second optical path,-   g) a portion of the second optical path light incident at the point    on the platen contacted by the external material being transmitted    out of the platen, while portions of said second optical path light    not incident at the point are reflected downwards in a third optical    path, the third optical path traversing one core,-   h) an active SBG column element of the second SBG array along the    third optical path diffracting the third angle light in a second TIR    path down the traversed core and proceeding along a TIR path along    the core to the detector.

In one embodiment of the invention there is provided a contact imagesensor using a single SBG array layer comprising: an illumination meansfor providing a collimated beam of first polarisation light; an SBGarray device further comprising first and second transparent substratessandwiching an array of selectively switchable SBG columns, andtransparent electrodes applied to opposing faces of the substrates, saidSBG substrates together providing a first TIR light guide fortransmitting light in a first TIR beam direction; a first transmissiongrating layer overlaying the lower substrate of the SBG array device; asecond transmission grating layer overlaying the upper substrates of theSBG array device; a quarter wavelength retarder layer overlaying thesecond transmission grating layer; a platen overlaying thy quarterwavelength retarder layer; and a polarization rotating reflecting layeroverlaying the first transmission grating layer. The apparatus furthercomprises: means for coupling light from said illumination means intosaid SBG array device; means for coupling light out of the second SBGarray device into an output optical path; and a detector comprising atleast one photosensitive element.

In one embodiment of the invention a contact image sensor comprises: anillumination means for providing a collimated beam of first polarizationlight; an illuminator waveguide for propagating light in a first TIRpath containing a first array of switchable grating columns; a detectorwaveguide for propagating light in a second TIR path containing a secondarray of switchable grating columns; a beam steering means comprising atleast one grating disposed between the platen and the detectorwaveguide; a first waveguide coupler for coupling light from theillumination means into the illuminator waveguide; a second waveguidecoupler for coupling light out of the detector waveguide into an outputoptical path; a detector comprising at least one photosensitive element;and a platen. Each switchable grating element in the first and secondswitchable grating arrays is switchable between a diffracting state anda non-diffracting state. The switchable grating elements diffract onlythe first polarization light. Each external surface of the detectorwaveguide is divided into a first grid of strips interspersed with asecond grid of strips. The first and second grids have differentlight-modifying characteristics. Overlapping strips from the first gridof strips on each external surface are operative to waveguide light.Overlapping strips from the second grid of strips on each externalsurface are operative to absorb light scattered out of regions of thedetector waveguide sandwiched by overlapping strips from the first gridof strips on each external surface. The strips are orthogonal to theswitchable grating columns.

In one embodiment the first grid of each external waveguide surface isone of clear or scattering and the second grid of at least one externalwaveguide surface is infrared absorbing.

In one embodiment the beam steering means comprises: a firsttransmission grating layer; a half wavelength retarder layer overlayingthe first transmission grating layer; a second transmission gratinglayer overlaying the half wavelength retarder layer; and a quarterwavelength retarder layer sandwiched by the second transmission gratinglayer and the platen.

In one embodiment the external faces of the detector waveguide and theilluminator waveguide abut an air space or a low refractive indexmaterial layer.

In one embodiment the first waveguide coupler couples light from theillumination means into the first TIR path in the illuminator waveguide.A switchable grating element of the illuminator waveguide in adiffracting state diffracts the first TIR path light towards the plateninto a first beam direction. The beam steering means deflects the firstbeam direction light towards the platen in a second beam direction. Whencontact is made with an external material at a point on the platen aportion of the second beam direction light incident at the point on theplaten contacted by the external material is transmitted out of theplaten. Light incident on the outer surface of the platen in the absenceof the contact with an external material is reflected towards thedetector waveguide in a third optical path. An active column of thesecond switchable grating array along the third beam direction diffractsthe third angle light into a second TIR path in the detector waveguide.The second waveguide coupler couples the second TIR path light into anoutput optical path towards the detector. In one embodiment the first tothird optical paths and the first and second TIR paths are in a commonplane. In one embodiment the first direction light traverses thedetector waveguide. In one embodiment the second direction lighttraverses the illuminator waveguide.

In one embodiment a method of making a contact image measurement isprovided comprising the steps of:

-   a) providing an apparatus comprising: an illumination means for    providing a collimated beam of first polarisation light; an    illuminator waveguide for propagating light in a first TIR beam    direction containing a first array of switchable grating columns; a    detector waveguide for propagating light in a first TIR beam    direction containing a second array of switchable grating columns; a    beam steering means comprising at least one grating disposed between    the platen and the detector waveguide; a first waveguide coupler for    coupling light from the illumination means into the illuminator    waveguide; a platen; a second waveguide coupler for coupling light    out of the detector waveguide into an output optical path; and a    detector comprising at least one photosensitive element. The    external surfaces of the detector waveguide comprise interspersed    multiplicities of strips with different light modifying    characteristics. The strips are orthogonal to the switchable grating    columns, each light modifying strip overlapping a clear strip;-   b) coupling light from the illumination means into the illuminator    waveguide;-   c) an external material contacting a point on the external surface    of the platen;-   d) sequentially switching elements of the first switchable grating    array into a diffracting state, all other elements being in their    non-diffracting states;-   e) sequentially switching columns of the second switchable grating    array into a diffracting state, all other columns being in their    non-diffracting states;-   f) each diffracting switchable grating element of the first    switchable grating array diffracting incident first TIR light    upwards into a first optical path;-   g) the beam steering means deflecting the first optical path light    into a second optical path;-   h) a portion of the second optical path light incident at the point    on the platen contacted by the external material being transmitted    out of the platen, portions of the second optical path light not    incident at the point being reflected into a third optical path;-   i) an active switchable grating column element of the second    switchable grating array along the third optical path diffracting    the third angle light in a second TIR path; and-   j) coupling light out of the detector waveguide towards the    detector.

In one embodiment the first to third optical paths and the first andsecond TIR paths are in a common plane.

A more complete understanding of the invention can be obtained byconsidering the following detailed description in conjunction with theaccompanying drawings wherein like index numerals indicate like parts.For purposes of clarity, details relating to technical material that isknown in the technical fields related to the invention have not beendescribed in detail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side elevation view of a contact image sensor in afirst embodiment of the invention.

FIG. 2 is a schematic front elevation of the waveguiding structure usedin the first embodiment of the invention showing the cross sections ofthe waveguide cores and cladding.

FIG. 3A is a schematic plan view of a first operational state of an SBGdevice used in a first embodiment of the invention.

FIG. 3B is a schematic plan view of a second operational state of an SBGdevice used in a first embodiment of the invention.

FIG. 4 is a schematic side elevation view of a contact image sensor in afirst embodiment of the invention showing the principle ray paths.

FIG. 5A is a schematic side elevation view of a detail of the contactimage sensor showing the ray propagation through the waveguide core andsecond SBG array in one embodiment of the invention.

FIG. 5B is a schematic side elevation view of a detail of the contactimage sensor showing the ray propagation through the waveguide core andsecond SBG array in one embodiment of the invention.

FIG. 6 is a schematic plan view of a wave-guiding structure and detectormodule used in one embodiment of the invention.

FIG. 7 is a schematic plan view of a wave-guiding structure and detectormodule used in one embodiment of the invention.

FIG. 8 is a schematic plan view of a wave-guiding structure and detectormodule used in one embodiment of the invention.

FIG. 9 is a schematic side elevation view of a detection scheme based onterminating waveguides in the wave-guiding structure with an angledpolished facet as used in one embodiment of the invention.

FIG. 10 is a schematic side elevation view of a detection scheme basedon applying out coupling gratings to waveguides in the wave-guidingstructure as used in one embodiment of the invention.

FIG. 11 is a schematic plan view of a detection scheme based on a twodimensional array used in one embodiment of the invention.

FIG. 12A is a schematic side elevation view of an illumination means inone embodiment of the invention.

FIG. 12B is a schematic plan view of an illumination means in oneembodiment of the invention.

FIG. 13 is a schematic plan view of an illumination means in oneembodiment of the invention.

FIG. 14 is a flow chart illustrating a method of making a contact imagemeasurement in one embodiment of the invention

FIG. 15 is a schematic side elevation view of a contact image sensor inone embodiment of the invention.

FIG. 16 is a schematic side elevation view of a contact image sensor inone embodiment of the invention showing the principle ray paths.

FIG. 17 is a schematic side elevation view of a contact image sensor inone embodiment of the invention.

FIG. 18 is a table showing typical refractive indices and layerthicknesses used in the first embodiment of the invention.

FIG. 19 is a schematic side elevation view of a contact image sensor inone embodiment of the invention.

FIG. 20 is a schematic diagram showing the key components a contactimage sensor in one embodiment of the invention.

FIG. 21 is a schematic side elevation view of a detector waveguide inone embodiment of the invention.

FIG. 22 is a schematic side elevation view of a detector waveguide inone embodiment of the invention showing the coupling of signal light viaan active element of the SBHG array.

FIG. 23 is a schematic plan view of a wave-guiding structure anddetector module used in one embodiment of the invention.

FIG. 24 is a cross-sectional view showing a detail of a detectorcomponent using a SBG waveguiding structure in one embodiment of theinvention.

FIG. 25 is a plan view of the SBG switching electrodes used in one layerof a detector component based a SBG waveguiding structure in oneembodiment of the invention.

FIG. 26 is a plan view of the SBG switching electrodes used in one layerof a detector component based a SBG waveguiding structure in oneembodiment of the invention.

FIG. 27 is a side elevation view of a contact image sensor in oneembodiment of the invention in which the detector and illuminatorcomponents are performed by a single waveguide containing a single SBGarray.

FIG. 28A is a side elevation view of a contact image sensor in oneembodiment of the invention in which external surfaces of the detectorwaveguide are divided into interspersed grids of strips having differentlight-modifying characteristics to provide a multiplicity of parallelwaveguiding paths.

FIG. 28B is a detail of the embodiment of FIG. 28A showing theinterspersed grid of strips on a first external surface.

FIG. 28C is a detail of the embodiment of FIG. 28A showing theinterspersed grid of strips on a first external surface.

FIG. 28D is a detail of the embodiment of FIG. 28A showing a crosssection of the detector waveguide with beam cross sections.

FIG. 29 is a side elevation view of the detector waveguide in theembodiment of FIG. 28A showing a side view of the SBG array and theinterspersed grids of strips applied to the external surface.

FIG. 30 is a front elevation view of the detector waveguide in theembodiment of FIG. 28A showing a cross section of the SBG array.

FIG. 31A shows an alternative configuration of the strips on a firstexternal surface of the detector waveguide of FIG. 28A.

FIG. 31B shows an alternative configuration of the strips on a secondexternal surface of the detector waveguide of FIG. 28A.

FIG. 32 shows alternative of strip configurations that may be used onthe external surfaces of the detector waveguide of FIG. 28A.

FIG. 33 is a schematic three dimensional view showing the platen anddetector waveguide in one embodiment in which the detector waveguide iscoupled to the detector by means of a micro lens array.

FIG. 34 is a schematic three dimensional view showing the platen anddetector waveguide in one embodiment in which the detector waveguide isdirectly coupled to the detector.

FIG. 35 is a flow chart illustrating a method of making a contact imagemeasurement using the apparatus of FIG. 28A.

FIG. 36A is a side elevation view of a contact image sensor in oneembodiment of the invention in which the detector comprises a SBG arrayand a waveguide array and external surfaces of the waveguide array isdivided into interspersed grids of strips having differentlight-modifying characteristics to provide a multiplicity of parallelwaveguiding paths.

FIG. 36B is a detail of the embodiment of FIG. 36A showing a plan viewof the interspersed grid of strips on the external surface.

FIG. 36C is a cross sectional view of the waveguide array in theembodiment of FIG. 36A.

FIG. 37A is a plan view of a first operational state of a twodimensional SBG array used in the at least one of the detector andilluminator waveguides in one embodiment.

FIG. 37B is a plan view of a second operational state of a twodimensional SBG array used in the at least one of the detector andilluminator waveguides in one embodiment of the invention.

FIG. 38 is a block diagram illustrating the key system modules of asoftware platform for use with a contact image sensor for finger printsensing in one embodiment of the invention.

DETAILED DESCRIPTION

It will be apparent to those skilled in the art that the presentinvention may be practiced with some or all of the present invention asdisclosed in the following description. For the purposes of explainingthe invention well-known features of optical technology known to thoseskilled in the art of optical design and visual displays have beenomitted or simplified in order not to obscure the basic principles ofthe invention.

Unless otherwise stated the term “on-axis” in relation to a ray or abeam direction refers to propagation parallel to an axis normal to thesurfaces of the optical components described in relation to theinvention. In the following description the terms light, ray, beam anddirection may be used interchangeably and in association with each otherto indicate the direction of propagation of light energy alongrectilinear trajectories.

Parts of the following description will be presented using terminologycommonly employed by those skilled in the art of optical design.

It should also be noted that in the following description of theinvention repeated usage of the phrase “in one embodiment” does notnecessarily refer to the same embodiment.

In the following description the term “grating” will refer to a Bragggrating. The term “switchable grating” will refer to a Bragg gratingthat can be electrically switched between an active or diffracting stateand an inactive or non-diffractive state. In the embodiments to bedescribed below the preferred switchable grating will be a SwitchableBragg Grating (SBG) recording in a Holographic Polymer Dispersed LiquidCrystal (HPDLC) material. The principles of SBGs will be described inmore detail below. For the purposes of the invention a non switchablegrating may be based on any material or process currently used forfabricating Bragg gratings. For example the grating may be recorded in aholographic photopolymer material.

An SBG comprises a HPDLC grating layer sandwiched between a pair oftransparent substrates to which transparent electrode coatings have beenapplied. The first and second beam deflectors essentially compriseplanar fringe Bragg gratings. Each beam deflector diffracts incidentplanar light waves through an angle determined by the Bragg equation toprovide planar diffracted light waves.

An (SBG) is formed by recording a volume phase grating, or hologram, ina polymer dispersed liquid crystal (PDLC) mixture. Typically, SBGdevices are fabricated by first placing a thin film of a mixture ofphotopolymerizable monomers and liquid crystal material between parallelglass plates. Techniques for making and filling glass cells are wellknown in the liquid crystal display industry. One or both glass platessupport electrodes, typically transparent indium tin oxide films, forapplying an electric field across the PDLC layer. A volume phase gratingis then recorded by illuminating the liquid material with two mutuallycoherent laser beams, which interfere to form the desired gratingstructure. During the recording process, the monomers polymerize and theHPDLC mixture undergoes a phase separation, creating regions denselypopulated by liquid crystal micro-droplets, interspersed with regions ofclear polymer. The alternating liquid crystal-rich and liquidcrystal-depleted regions form the fringe planes of the grating. Theresulting volume phase grating can exhibit very high diffractionefficiency, which may be controlled by the magnitude of the electricfield applied across the PDLC layer. When an electric field is appliedto the hologram via transparent electrodes, the natural orientation ofthe LC droplets is changed causing the refractive index modulation ofthe fringes to reduce and the hologram diffraction efficiency to drop tovery low levels resulting in for a “non diffracting” state. Note thatthe diffraction efficiency of the device can be adjusted, by means ofthe applied voltage, over a continuous range from near 100% efficiencywith no voltage applied to essentially zero efficiency with asufficiently high voltage applied. U.S. Pat. No. 5,942,157 and U.S. Pat.No. 5,751,452 describe monomer and liquid crystal material combinationssuitable for fabricating SBG devices.

To simplify the description of the invention the electrodes and thecircuits and drive electronics required to perform switching of the SBGelements are not illustrated in the Figures. Methods for fabricatedpatterned electrodes suitable for use in the present invention aredisclosed in PCT US2006/043938. Other methods for fabricating electrodesand schemes for switching SBG devices are to be found in the literature.The present invention does not rely on any particular method forfabricating transparent switching electrodes or any particular schemefor switching arrays of SBGs. Although the description makes referenceto SBG arrays the invention may be applied using any type of switchablegrating.

To clarify certain geometrical of aspects of the invention referencewill be made to the orthogonal XYZ coordinate system where appropriate.

A contact image sensor according to the principles of the invention isillustrated in the schematic side elevation view of FIG. 1. Theapparatus comprises the following parallel optical layers configured asa stack: an illumination means 1 for providing a collimated beam offirst polarized light; a first SBG array device 2 further comprisingfirst and second transparent substrates 21,22 sandwiching an array 20 ofselectively switchable SBG column elements, and ITO electrodes 20A,20Bapplied to opposing faces of the substrates, the SBG substrates togetherproviding a first TIR light guide for transmitting light in a first TIRbeam direction; an air gap 23; a transmission grating 43; a thirdtransparent substrate(low index glue layer 42; a low refractive indexSBG cover glass 41; a ITO layer 40B; a second SBG array device 4comprising an array of selectively switchable SBG column elements; a ITOlayer 40B; a barrier film 40C; a waveguiding layer 50 comprising amultiplicity of waveguide cores separated by cladding material having agenerally lower refractive index than the cores, the cores beingdisposed parallel to the first beam direction; an upper clad layer 51having a generally lower refractive index than the cores(which is alsoreferred to as the bottom buffer); a priming layer 61; and a platen 6.Each core of the waveguide structure is optically couple to an elementof a detector array. The details of the waveguide to detector couplingwill be discussed later. The apparatus further comprises: means forcoupling light from the illumination means into the first TIR lightguide; means for coupling light out of the core into an output opticalpath; and a detector comprising at least one photosensitive element, thephotosensitive element being optically coupled to at least one the core.The illumination means may further comprise optical stops to eliminatestray light and scatter. The first polarized light may be either S or Ppolarized. Since SBGs recorded in the inventors preferred HPDLC materialsystem are P-polarization sensitive that polarization will be assumedfor the purposes of describing he invention. The transmission grating 43is advantageously a conventional transmission Bragg grating recorded ina holographic photopolymer. However, other equivalent means forproviding a transmission grating may be used. Desirably, the contactimage sensor uses infrared light from at least one laser. In oneembodiment of the invention the light wavelength is 785 nanometers. Across sectional view (in the XZ plane) of the waveguiding structure isshown in FIG. 2 which illustrates the waveguiding structure 40sandwiched by the barrier film 40C and the clad layer 51 (or bottombuffer). A core 71 and a region of cladding 72 between adjacent cores isindicted in the drawing.

In functional terms the first SBG device 20 comprises an array of stripsor columns aligned normal to the light propagation direction of the TIRlight. The second SBG array also comprises an array of strips or columnsaligned parallel to the strips in the first SBG device. The SBGs in thefirst and second SBG arrays are recorded as single continuous element ineach case. Transparent electrodes are applied to the opposing surfacesof the substrates 21,22 with at least one electrode being patterned todefine the SBG elements. As explained above each SBG element in thefirst and second SBG arrays has a diffracting state when no electricfield is present across the ITO electrodes and a non-diffracting statewhen an electric field is present across the ITO electrodes, the SBGelements diffracting only the first polarization light. Transparentelectrodes are applied to the opposing faces of the third transparentsubstrate and the waveguiding layer with at least one electrode beingpatterned to define the SBG elements. Typically the first SBG array hasa resolution of 1600 elements. The resolution of the second SBG array islower, typically 512 elements.

The column elements of the first and second SBG arrays have longerdimensions disposed orthogonally to the first TIR beam direction. Theelements of the second SBG device which are in a non-diffracting statehave a generally lower refractive index than the waveguide cores. Thethird transparent substrate has a generally lower refractive index thanthe cores. At any time one element of the first SBG array is in adiffracting state, one element of the second SBG array is in adiffracting state, all other elements of the first and second SBG arraysare in a non-diffracting state.

In the embodiment illustrated in FIG. 1 all of the above describedlayers (apart from the air gap 23 between the upper substrate 21 of thefirst SBG and the transmission grating 43) are in contact, forming alaminated structure. It should be noted that the relative dimensions ofthe various layers are greatly exaggerated in the drawing. In oneembodiment of the invention the air gap 23 may be replace by arefracting material layer. The second SBG array 4 acts as the lowercladding layer of the wave guiding structure while the waveguide core 50and the third transparent substrate 41 act as the containing substratesof the second SBG array device 4. The first and second transparentsubstrates 21,22 sandwiching the first SBG array together provide afirst TIR light guide with the TIR occurring in the plane of thedrawing. The second SBG array device 4 is sandwiched by the waveguidecore and the third transparent substrate 41 which form a second TIRlight guide.

The contact image sensor further comprises a means 11 for coupling lightfrom said illumination means 1 into the first SBG array lightguide. Theinvention does not assume any particular coupling means. One particularsolution discussed later is based on prismatic elements. In oneembodiment the coupling means may be based on gratings. The contactimage sensor further comprises a means for coupling light out of thewave-guiding structure into an output optical path leading to adetector. The coupling means which schematically represented by thedashed line 52 is advantageously a grating device which will bediscussed in more detail later.

The column elements of the first and second SBG arrays are switchedsequentially in scrolling fashion, backwards and forwards. In each SBGarray the SBG elements are switched sequentially across the SBG arrayand only one SBG element in each array is in its diffracting state atany time. The effect is to produce a narrow scanning column of lightthat sweeps backwards and forwards across the platen. The disposition ofthe SBG elements in the first SBG array is illustrated in FIGS. 3A-3Bwhich provides schematic plan views of the SBG array 20 at twoconsecutive switching states. In the first state illustrated in FIG. 3Athe SBG element indicated by 25 is in its diffracting state and allother SBG elements are in their non diffracting states, allowing TIRlight to be transmitted through the arrays without substantialtransmission loss or path deviation. In the second state illustrated inFIG. 3B the SBG element 24 is switched to its non-diffracting statedwhile the adjacent element 25 is switched to its diffracting state.

We next discuss the operation of the device with reference to theschematic side elevation views of FIG. 4-5. By considering the path ofP-polarized collimated light through the device in the plane of eachdrawing. Incident light 200 from the illuminator means 1 is coupled intothe first SBG device 2 by a coupling means indicated by 11 which will bediscussed below. The light undergoes TIR in the light guide formed bythe substrates 21,22 as indicted by the rays 201-203. The active (i.e.diffracting) SBG column element 23 diffracts light 204 out of the lightguide The light 204 is now diffracted by the transmission grating intothe ray 206 which propagates towards the platen without significantdeviation or loss through the intervening optical layers. The symbol Pindicates that the light is P-polarized, i.e. it retains thepolarization of the input laser light.

During a scan the fingers are placed onto the scanner surface. In theabsence of finger contact the light incident on the platen outer surfaceis totally internally reflected downwards towards the wave guidingstructure 50 and then on to the detector. When finger contact is madethe finger skin touching the platen surface causes reflection at theouter surface of the platen to be frustrated such that light leaks outof the platen. The parts of the finger skin that touch the platensurface therefore becomes the dark part of the finger print imagebecause light never makes it to the detector array. The X coordinate ofthe contacting feature is given by the detector array element providingthe dark-level or minimum output signal. The latter will be determinedby the noise level of the detector. The Y coordinate of the contactingfeature is computed from the geometry of the ray path from the last SBGelement in the first SBG array that was in a diffracting state justprior to TIR occurring in the platen and a signal from the reflectedlight being recorded at the detector. The ray path is computed using thediffraction angle and the thicknesses and refractive indices of theoptical layers between the SBG element and the platen surface.

In one embodiment of the invention an alternative detection scheme isbased on the principle that in the absence of any external pressure artthe platen/air interface the incident light is transmitted out of theplaten. Now, external pressure from a body 62 of refractive index lowerthan the platen (which may a feature such as a finger print ridge orsome other entity) applied on the outer side of the platen layer causesthe light to be totally internally reflected downwards towards the waveguiding structure 50. Hence the X coordinate of the contacting featureis now given by the detector array element providing the peak outputsignal. The procedure for computing the Y coordinate remains unchanged.

An SBG when in the state designated as “non-diffracting” will, inpractice, have a very small refractive index modulation and willtherefore diffract a small amount of light. This residual diffraction isnegligible in most applications of SBGs. However, in applications suchas the present invention any residual refractive modulation will resultin a small amount of light being diffracted out of the light guide. Forexample referring to FIG. 4, SBG elements such as 24 will have a smalldiffraction efficiency leading to a small portion of TIR light beingdiffracted upwards into the ray path represented by the dashed lines andthe ray directions indicated by 220-223. This light will follow aparallel oath to the light from the active SBG element (the signallight) and will be reflected off the platen outer surface towards thewaveguides. Coupling of this stray light into the waveguides, where itwill contribute a background leakage noise to the output signal, isprevented by switching the second SBG array elements in synchronizationwith the first array elements such that only the element of the firstand second SBGs array lying on the signal ray path are in a diffractingstate at any time. The readout of the signal from detector array is inturn synchronized with the switching of the elements of the first andsecond' SBG arrays.

The wave guiding structure 50 and the SBG array 4 together provide themeans for coupling light out of the sensor onto a detector array. TheSBG provides the lower cladding and the layer 51 provides the uppercladding. The coupling of light into the waveguide relies on the secondSBG array which acts as a switchable cladding layer as will be discussedbelow. The second SBG array is operated in a similar fashion to thefirst SBG array with column elements being switched sequentially inscrolling fashion, backwards and forwards. Only one SBG element is in adiffracting state at any time. The non active elements perform thefunction of a clad material. The role of the active SBG element is tosteer incident ray into the TIR angle. It should be appreciated that inorder that light reflected down from the platen can be diffracted into aTIR path by an active (diffracting) SBG element the refractive index ofthe SBG in its active state must be lower than the core index. Tomaintain TIR the refractive index of the SBG elements that are not intheir diffracting states must be lower than that of the core. Theoperation of the waveguiding structure will now be explained moreclearly referring to FIG. 5A which shows a detail of the wave guidingstructure including the cladding 51, core 50, second SBG array 4 and SBGsubstrate 41. Note that in FIGS. 5A-5B the layers 40A,40B,40C are notillustrated. For the sake of simplifying the description the refractionof light at the optical interfaces will be ignored. The SBG grating isrepresented by the single Bragg fringe 44. The ray 207 on entering theactive SBG element 43 at an incidence angle w is diffracted into the ray207A. The deflection of the ray is determined by the Bragg diffractionequation. Since the average index of the SBG medium is higher than thatof the substrate layer 41 the diffracted ray 207A undergoes TIR withinthe SBG medium and the reflected ray 208 propagates into the core at anangle u which is slightly higher than the critical angle of thecore/cladding interface. The angle u is determined by the slant anglesof the Bragg fringes and the incidence angle w. The ray 208 undergoesTIR to give the downward ray 209 which enters the non diffracting SBGelement 45 at the angle u as the ray 210. The ray 210 undergoes TIR atthe interface of the SBG element/third substrate and re-enters the coreas the ray 211 which from reflection symmetry is at angle u. Thisprocess is repeated along the waveguide until the light is coupled outtowards the detector. Since all of the remaining SBG elements along thewaveguide path are in their non diffracting states TIR between thecladding layer and the SBG lower substrate continues until the light iscouple out of the waveguide towards the detector.

The invention also covers the case where the SBG substrate abuts a lowindex slab 42 which has a lower index than the third substrate. Thelayer 42 is not essential in all applications of the invention but willin general provide more scope for optimizing the optical performance ofthe sensor. Referring to FIG. 5B it will be seen that the ray paths aresimilar to those of FIG. 5A except that the TIR of the diffracted ray207A now takes place at the interface between the substrate 41 and thelow index slab 42. Accordingly, the diffracted ray 207A is transmittedinto the substrate 41 as the ray 207B and undergoes TIR into the ray207C at the low index layer after transmission through the substrate 41and the SBG array 4 the ray now indicated by 208A propagates into thecore at an angle v which is slightly higher than the critical angle ofthe core/clad interface. The ray 208A undergoes TIR to give the downwardray 209A which enters the non activated SBG element 45 as the ray 210A.The ray 210A undergoes TIR at the low index layer and re-enters the coreas the ray 211A which from reflection symmetry is at angle v. Thisprocess is repeated along the waveguide until the light is coupled outtowards the detector. It should be appreciated that in situations wherethe collimation of the beam is not very tightly controlled it ispossible that TIR may occur at the SBG substrate index for some rays andat the low index slab substrate for other rays.

In one embodiment of the invention the third transparent substrate has agenerally lower refractive index than an element of the second SBG arrayin its diffracting state.

In one embodiment of the invention the third transparent substrate has agenerally lower refractive index than the element of the second SBGarray in its non-diffracting state.

As indicated in FIGS. 5A-5B the cleared SBG will still have a smallresidual refractive index modulation which causes a small amount of theincident light to be diffracted. The direction of diffraction willdepend on the TIR angle. In some cases the ray may not be at the Braggangle but may still be sufficiently close to the Bragg angle to bediffracted, but with a lower diffraction efficiency. If not diffractedit may end up in the TIR beam, thereby contributing to the outputsignal.

Turning back to FIGS. 1-2 we see that the wave-guiding structure 50which is illustrated in schematic plan view in FIG. 1 and in crosssection in FIG. 2 comprises a multiplicity of parallel strip waveguidesgenerally indicated by 70, the waveguide core element of one of thewaveguides and the surrounding cladding being indicated by numerals71,72. The invention does not assume any particular waveguide geometryor material for fabricating the waveguiding layer. It should be apparentto those skilled in the art of integrated optics that a large number ofdifferent core/cladding combinations may be used in the invention.Typically, the core will have a refractive index of typically between1.51 to 1.56 or and the cladding layers will have refractive indices inthe range from 1.41 to 1.47. Typically the core may be rectangular withcross sectional dimensions of 25-40 microns in depth×40 microns inwidth. However, the cores may have much larger or much smaller crosssectional dimensions subject to the specifications for couplingefficiency, waveguide crosstalk and other waveguide parameters set bythe application. The wave-guiding structure may use a polymer waveguidecore of index typically in the range 1.50 to 1.60 with cladding indextypically 1.45 to 1.55. However, the invention does not assume anyparticular waveguide optical materials. It should be noted by thewaveguide cladding in the waveguiding layer 51 and the cladding layer 51may be fabricated from one material. In some cases it may be advantagesto have more than one cladding material in order to provide bettercontrol of the guide wave mode structure. The highest refractive indexUV curable material suitable for use as either core or cladding in ahigh transparency waveguiding structure of the type required in theinvention is believed to have a refractive index of about 1.56 at 633nm. The index might be slightly lower at longer wavelength. The problemwith index values above about 1.56 is that the materials become eithercolored or slightly metallic and hence lose their transparency. Higherindex transparent materials exist but they are not UV curable, whichmakes them unsuitable for waveguide fabrication using currentlyavailable embossing process.

We next discuss the means for coupling light out of the wave-guidingstructure into an output optical path leading to a detector. Thecoupling scheme which was only indicated schematically by the symbol 52in FIG. 1 may be based on well-known methods using grating couplers,prismatic elements etc. The invention does not rely on any particularmethod. FIGS. 6-8 provides schematic plan views of alternative schemesfor coupling the wave guiding structure 50 to the detector 8. Thedetector comprises at least one element. A multiplicity of waveguidecores is generally indicated by 70 with a typical core element 71 andthe surrounding cladding 71 being indicated in each case. Each coreterminates at a coupler linked to a detector element. In each case theray paths from the active SBG element 23 to the waveguide terminationare indicated by 206,207,213 using the numerals of FIG. 4. In theembodiment of FIG. 6 the detector 8 is a linear array of elements suchas 81. A ray path from the waveguide termination to the detector isindicated by 214. Advantageously, the cores are each terminated by a 45degree facet with directs light upwards or downwards (relative to thedrawing surface) towards the detector along direction 214 which shouldbe read as normal to the plane of the drawing. The detector pitchmatches the core spacing. In one embodiment of the invention a parallelpath waveguide routing element may be provided between the waveguidetermination and the detector. In the embodiment of FIG. 7 the outputlight paths generally indicated by 502 from the waveguides are convergedonto a linear detector array that is much smaller than the width of theplaten by means convergent path waveguide routing element 84A. In oneembodiment of the invention the cores are terminated by a 45 degreefacet which directs the light upwards or downwards. In the embodiment ofFIG. 8 the output light paths generally indicated by 503 from thewaveguides are converged by means of a convergent path waveguide routingelement 85 onto a single element detector 83. In one embodiment of theinvention the cores are terminated by a 45 degree facet which directsthe light upwards or downwards.

Many different schemes for providing the waveguiding routing elementsreferred to above will be known to those skilled in the art ofintegrated optical systems. The apparatus may further comprise a microlens array disposed between the waveguide ends and the detector arraywhere the micro lens elements overlap detector elements. FIG. 9 is aschematic side elevation view of one method of coupling light out of thewave-guiding structure in which there is provided a 45 degree facet 86Aterminating each waveguide element in the wave-guiding structure. FIG. 9may be a cross section of any of the schemes illustrated above. Thedetector 8 and the waveguide cladding layers 75,76 and core 74 areillustrated. The core 74 may be a continuation of one of the cores 70 ora core of material of similar optical properties optically coupled toone of said cores 70. The cladding layer may be a continuation of thecladding layer 51 in FIG. 1 and FIG. 3 or material of similar refractiveindex. The cladding layer may be continuation of the HPDLC material ofthe SBG array 4 or material of similar refractive index to the SBG arrayin its non-active state. FIG. 10 is a schematic side elevation view ofanother method of coupling light out of the wave-guiding structure inwhich a grating device 86B is applied to each waveguide element. FIG. 10may be a cross section of any of the schemes illustrated in FIGS. 7-9.The grating may be a surface relief structure etched into the waveguidecladding. Alternatively, the grating may be a separate layer in opticalcontact with one or both of the core or cladding. In one embodiment ofthe invention the grating may be recorded into a cladding layer as aBragg grating.

In the above described embodiments of the invention the detector 8 is alinear array. In an alternative embodiment of the invention illustratedin FIG. 11 the detector elements are distributed over two dimensions.This avoids some of the alignment problems of coupling waveguideelements to detector elements with a very high resolution linear array.The waveguides from the wave-guiding structure generally indicated by 87are fanned out in the waveguide groups 87A,87C. The detectors aregenerally indicated by 88. The waveguide groups 87A,87C containwaveguide cores such as 87B which overlays the detector 88B in thedetector group 88A and waveguide 87D which overlays the detector 88D inthe detector group 88C. The waveguide to detector computing may employ45 degrees core terminations, gratings, prisms or any other methodsknown to those skilled in the art. From consideration of FIG. 11 itshould be apparent that many alternative configurations for coupling thewaveguiding structure to a two dimensional detector array are possible.

In practical embodiments of the invention the beams produced by theillumination means will not be perfectly collimated even with smalllaser die and highly optimized collimating optics. For this reason theinteractions of the guided beams with the SBG elements will not occur atthe optimum angles for maximum Bragg diffraction efficiency (DE) leadingto a small drop in the coupling efficiency into the waveguidingstructure. Having coupled light into the waveguiding structures there isthe problem that some of the light may get coupled out along the TIRpath by the residual gratings present in the non diffracting SBGelements. The reduction in signal to noise ratio (SNR) resulting fromthe cumulative depletion of the beam by residual gratings along the TIRpath in the output waveguide may be an issue in certain applications ofthe invention. A trade-off may be made between the peak and minimum SBGdiffraction efficiencies to reduce such out-coupling. The inventors havefound that minimum diffraction efficiencies of 0.02% are readilyachievable and efficiencies as low as 0.01% are feasible. To furtherreduce the risk of light being coupled out of the waveguiding structure,a small amount of diffusion (−0.1%) can be encoded into the SBG toprovide a broader range of angles ensuring that guided light is not allat the Bragg angle. A small amount of diffusion will be provided byscatter within the HPDLC material itself. Further angular dispersion ofthe beam may also be provided by etching both the ITO and the substrateglass during the laser etching of the ITO switching electrode.

In one embodiment of the invention the refractive index modulation ofsecond SBG array is varied along the length of the array during exposureto provide more uniform coupling along the waveguide length. Therequired variation may be provided by placing a variable neutral densityfilter in proximity to the SBG cell during the holographic recording. Inany case the power depletion along the waveguide can be calibratedfairly accurately.

Only light diffracted out of the active element of the first SBG arrayshould be coupled into the output waveguide structure at any time. Inpractice the SBG array comprises a continuous grating with theindividual elements being defined by the electrode patterning. The gapsbetween the elements of the first SBG arrays should therefore be made assmall as possible to eliminate stray light which might get coupled intothe waveguiding layer reducing the SNR of the output signal. Ideally thegap should be not greater than 2 micron. The noise signal contributed bythe gaps is integrated over the area of an active column element of thesecond SBG array element while the light contributing to the usefulsignal is integrated over the simultaneously active column element ofthe first SBG array. An estimate of SNR can be made by assuming a commonarea for the first and second SBG arrays and making the followingassumptions: number of elements in the second SBG array: 512; number ofelements in first SBG array: 1600; SBG high diffraction efficiency: 95%;and SBG low diffraction efficiency: 0.2%. The SNR is given by [area ofsecond SBG array element x high diffraction efficiency/[area of SBGelement×low diffraction efficiency=[1600×0.95]/[52×0.02]=148. Desirably,the SNR should be higher than 100.

In one embodiment of the invention the transparent electrodes arefabricated from PDOT (poly ethylenedioxythiophene) conductive polymer.This material has the advantage of being capable of being spin-coatedonto plastics. PDOT (and CNT) eliminates the requirement for barrierfilms and low temperature coating when using ITO. A PDOT conductivepolymer can achieve a resistivity of 100 Ohm/sq. PDOT can be etchedusing Reactive Ion Etching (RIE) processes.

In one embodiment of the invention the first and second SBG arrays areswitched by using a common patterned array of column shaped electrodes.Each element of the second SBG array, which is of lower resolution thanthe first SBG array uses subgroups of the electrode array.

In one embodiment of the invention the waveguides are fabricated fromPDOT. The inventors believe that such a waveguide will exhibit highsignal to noise ratio (SNR).

In one embodiment of the invention the waveguides are fabricated fromCNT using a lift-off stamping process. An exemplary CNT material andfabrication process is the one provided by OpTIC (Glyndwr InnovationsLtd., St. Asaph, Wales, and United Kingdom).

In one embodiment of the invention the waveguide cores are conductivephotopolymer such as PDOT or CNT. Only the portions of the SBG arraylying directly under the waveguide cores are switched. This avoids theproblems of crosstalk between adjacent waveguide cores thereby improvingthe SNR at the detector.

In one embodiment of the invention used for finger print detection whichuses infrared light of wavelength 785 nm the TIR angle in the platendepends on the refractive indices of the platen glass and the thin layerof water (perspiration) between the subject's skin and the platen. Forexample, if the platen is made from SF11 glass the refractive index at785 nm is 1.765643, while the index of water at 785 nm is 1.3283. FromSnell's law the arc-sine of the ratio of these two indices (sin-1(1.3283/1.76564) gives a critical angle of 48.79°. Allowing for the saltcontent of perspiration we should assume an index of 1.34, whichincreases the critical angle to 49.37°. Advantageously, the TIR angle atthe platen should be further increased to 50° to provide for alignmenttolerances, fabrication tolerances, and water variations as well ascollimation tolerances too for less than perfect lenses and placementsof these parts. Alternatively, other materials may be used for theplate. It is certainly not essential to use a high index to achievemoisture discrimination. One could use an acrylic platen (index 1.49),for example, where the ray angle is in the region of 65°. In practice,however, the choice of platen material will be influenced by the need toprovide as large a bend angle as possible at the SBG stage. The reasonfor this is that higher diffraction efficiencies occur when the bendangle (i.e. the difference between the input angle at the SBG and thediffracted beam angle) is large. Typically bend angles in the region of20-25° are required.

In one embodiment of the invention the platen may be fabricated from alower refractive material such as Corning Eagle XG glass which has arefractive index of 1.5099. This material has the benefit of relativelylow cost and will allow a sufficiently high TIR angle to enable saltywater discrimination. Assuming the above indices for perspiration (saltwater) of 1.34 and water of 1.33 the critical angle for salt water is62.55777° and the critical angle for water of 61.74544°.

In one embodiment of the invention the indices of the SBG substrates andthe element 42 are all chosen to be 1.65 and the platen index is chosento be 1.5099. The material used in the low index layer 42 is equal inindex to the SBG substrates, or slightly lower. The TIR angle in the SBGlayer is 78 degrees. At this index value the diffracted beam angle withrespect to the surface normal within the upper SBG substrate will be 55degrees. For a TIR angle of 78 degrees in the SBG the effectivediffraction bend angle is 23 degrees. The TIR angle in the platen basedon the above prescription is 63.5 degrees allowing for typicalrefractive index tolerances (i.e. a 0.001 refractive index tolerance and0.3 degree minimum margin for glass tolerances).

The above examples are for illustration only. The invention does notassume any particular optical material. However, the constraints imposedby the need for perspiration discrimination and the bend angles that canbe achieved in practical gratings will tend to restrict the range ofmaterials that can be used. Considerations of cost, reliability andsuitability for fabrication using standard processes will furtherrestrict the range of materials.

The required refraction angles in any layer of the sensors can bedetermined from the Snell invariant given by the formula n.sin(U)=constant where n is the refractive index and U is the refractionangle. Typically the constant will be set by the value of the Snellinvariant in the platen. For example if the platen index is 1.5099 andthe critical angle is 63.5° the Snell invariant is1.5099×sin(63.5°)=1.351. The only exception to this rule will be thecases where diffraction occurs at elements of the SBG arrays or thetransmission grating where the change in angle will defined by therespective grating prescriptions.

In the embodiment of FIG. 1 there is an air gap between the first SBGarray 2 and the transmission grating. Other air gaps may be providedbetween other layers in the sensor architecture subject to therestrictions imposed by the Snell invariant and the diffraction bendangle as discussed above.

The invention requires tight control of refractive index and angletolerances to maintain beam collimation otherwise cross talk betweenadjacent waveguides may occur leading to output signal ambiguities.Index variations: of 0.001 may lead to TIR boundaries shifting by around0.25° for example. Angular tolerances are typically 0.1° intransmission. At reflection interfaces the angular error increases. Inthe worst case a ray will experience reflections off five differentsurfaces. Note that the TIR paths used in the sensor can typicallyundergo up to 18 bounces. The effects of a wedge angle in the substrateswill be cumulative. For example, a 30 seconds of arc wedge may lead to a0.3° error after 18 bounces. Desirably, the cumulative angular errorsshould allow a margin for TIR of at least 1°. Typical refractive indicesand layer thicknesses used in the embodiment of FIG. 1 are provided inthe table of FIG. 18.

FIG. 12 illustrates the illumination module of the contact image sensorin one embodiment in more detail. FIG. 12A is a schematic side elevationview showing the illumination means and the SBG device in one embodimentof the invention. FIG. 12B provides a side elevation view of the sameembodiment of the invention. The wave guiding structure is notillustrated in FIG. 12A. The illumination means comprises a multiplicityof lasers indicated by 13A-13D providing separate parallel illuminationmodules, each module comprising a pair of crossed cylindrical lensessuch as 16A,16B a light guide 17, transparent slabs 12,19 andtransparent substrate 13. The slabs 12,19 abut the first SBG array 2comprising the transparent substrates 21,22 sandwiching the SBG layer20. In one embodiment the lenses 16A,16B may be crossed cylindricallenses such that the first lens 16A collimates the input light 101A toprovide a first beam 102A that is collimated in a first plane and thesecond lens 16B collimates the beam 102A in the orthogonal plane toprovide a beam 103A collimated in a second plane orthogonal to the firstplane such that the resulting beam in the light guiding element 17 iscollimated in both beam planes. Advantageously, the lenses are ofrectangular cross section. The beams from the lasers 13A-13D areidentical and abut to form a continuous rectangular beam extending overan area substantially the same as the first SBG array in plan view. Thelightguide element 17 comprises a transparent slab with a planar inputsurface orthogonal to the beam direction and a reflecting surface 14 atan angle to the beam direction. The surface 14 reflects the beam 104Ainto the direction 105A orthogonal to 104A. Although the slab portions12 and 19 are illustrated as being air separated they may abut. The slab12 has a tilted reflecting surface 18 for directing light 106A into theSBG array device 2. In one embodiment of the invention the slab 12 hasan identical refractive index to the substrates 21,22 sandwiching theSBG array 20. The slab 19 essentially performs the function of a spacer.The slab 13 also acts as spacer. In one embodiment of the invention theslab 13 is coated with a polarization selective coating in the regionilluminated by the upward propagating light reflected off the mirrorsurface 14. The refractive index of the slab 19 is chosen to ensure thatrays such as 106A,107A entering the first SBG array device exceed thecritical angles for TIR within the light guide formed by the first SBGarray device. The reflective surfaces 14,18 essentially provide thecoupling means indicated schematically by the symbol 11 in FIG. 1. Itshould be apparent to those skilled in the art of optical design that inother embodiments of the invention other equivalent opticalconfigurations including diffractive optical surfaces may be used toperform the function of the surfaces 14 and 18. Typically, the SBG arrayan average refractive index of 1.55 in its non-diffractive state and1.62 when in a diffracting state. The substrates 21,22 have refractiveindices of 1.55. The slab 12 has an index of typically between 1.5 and1.7 to match the SBG substrates. The slab 19 is advantageously a polymermaterial of refractive index 1.49. The resulting critical angle in thefirst TIR light guide formed by the first array SBG device is thereforeapproximate 74 degrees.

In one embodiment illustrated in the schematic plan view of FIG. 13 theillumination means comprises a single laser 13E and a collimator lenssystem comprising the crossed cylindrical lenses 46 a,46 b. The saidillumination means provides a single collimated beam of rectangularcross section 104E.

A sensor according to the principles of the present application mayfabricated using the HPDLC material system and processes disclosed inPCT Application No.: PCT/GB2012/000680 entitled IMPROVEMENTS TOHOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTAL MATERIALS AND DEVICES whichis incorporated by reference herein in its entirety. The SBG substratesmay fabricated from polycarbonate, which is favored for its lowbirefringence. Two other currently available plastic substratesmaterials are a cyclic olefin copolymer (COC) manufactured by TOPASAdvanced Polymers and sold under the trade name TOPAS. The other was acyclic olefin polymer (COP) manufactured by ZEON Corporation and soldunder the trade names ZEONEX and ZEONOR. These materials combineexcellent optical properties (including high transmission and lowbirefringence) with excellent physical properties (including lowspecific gravity, low moisture absorption, and relatively high glasstransition temperature). The inventors have found that an adequatediffraction efficiency (i.e. ≧70%) can be obtained when using plasticsubstrates. The diffraction efficiency compares favorably with glass.The switching time of plastic SBG is also found to be sufficient toproduce satisfactory devices.

Transparent conductive (ITO) coatings applied to the above plastics havebeen found to be entirely satisfactory, where satisfactory is defined interms of resistivity, surface quality, and adhesion. Resistivity valueswere excellent, typically around 1000/square. Surface quality (i.e., thesize, number and distribution of defects) was also excellent. Observabledefects are typically smaller than 1 micron in size, relatively few innumber, and sparsely distributed. Such imperfections are known to haveno impact on overall cell performance. ITO suffers from the problem ofits lack of flexibility. Given the rugged conditions under some SBGdevices may operate, it is desirable to use a flexible TCC with aplastic substrate. In addition, the growing cost of indium and theexpense of the associated deposition process also raise concerns. Carbonnanotubes (CNTs), a relatively new transparent conductive coating, areone possible alternative to ITO. If deposited properly, CNTs are bothrobust and flexible. They can be applied much faster than ITO coatings,are easier to ablate without damaging the underlying plastic, andexhibit excellent adhesion. At a resistivity of 200 Ω/sq, the ITOcoatings on TOPAS 5013S exhibit more than 90% transmission. At aresistivity of 230 Ω/sq, the CNT coatings deposited on the samesubstrates material exhibited more than 85% transmission. It isanticipated that better performance will results from improvements tothe quality and processing of the CNTs

An adhesion layer is required to support the transparent conductivecoating. The inventors have found that the adhesion of ITO or CNTdirectly to plastics such as TOPAS and ZEONEX was poor to marginal. Theinventors have found that this problem can be overcome by means of asuitable adhesion layer. One exemplary adhesion layer is Hermetic TEC2000 Hard Coat from the Noxtat Company. This material has been found toyield a clear, mar-resistant film when applied to a suitably preparedplastic substrate. It can be applied by flow, dip, spin, or spraycoating. TEC 2000 Hard Coat is designed to give good adhesion to manythermoplastic substrates that are cast, extruded, molded or blow molded.When applied to TOPAS, ZEONEX or other compatible plastics, the strengthand break resistance provided by TEC 2000 is nearly as scratch andabrasion resistant as glass. Hermetic Hard Coat forms a transparent 3-6micron film on plastic surfaces. The Refractive index of the coating is1.4902. A sample of TOPAS plastic sheet coated with TEC 2000 Noxtatprotective Hard Coat is shown in FIG. 13. The next step in SBG cellproduction process is applying the TCC (ITO or CNT) to the hard coat.FIG. 14 shows Noxtat Hard Coat samples with additional ITO and CNTcoatings. The Hard Coat plays two roles in SBG cell production. One isto increase adhesion of the conductive layer to the plastic and preventdegassing during vacuum coating. The second role is to seal the plasticsurface from environmental influence. It was found that TEC 2000 HardCoat performs very well with TOPAS and ZEONEX materials.

A fundamental feature of SBGs fabricated using current HPDLC materialsystems is that the grating is present when the device is in its passivestate. An electric field must be applied across the HPDLC layer to clearthe grating. An alternative HPDLC material system that may be used withthe present invention provides a reverse mode SBG in which the gratingis clear when in its passive state. A reverse mode SBG will providelower power consumption. Reverse mode SBG devices are disclosed in PCTApplication No.: PCT/GB2012/000680.

A method of a method of making a contact image measurement in oneembodiment of the invention in accordance with the basic principles ofthe invention is shown in the flow diagram in FIG. 14. Referring to theflow diagram, we see that the said method comprises the following steps:

At step 501 providing an apparatus comprising the following paralleloptical layers configured as a stack: an illumination means forproviding a collimated beam of first polarization light; a first SBGarray device further comprising first and second transparent substratessandwiching an array of selectively switchable SBG column elements, andITO electrodes applied to opposing faces of the substrates and the SBGsubstrates together providing a first TIR light guide for transmittinglight in a first beam direction; an air gap; a transmission grating; athird transparent substrate(low index glue layer); a SBG cover glass; aITO layer; a second SBG array device comprising array of selectivelyswitchable SBG column elements; a ITO layer; a barrier film; awaveguiding layer comprising a multiplicity of waveguide cores separatedby cladding material having a generally lower refractive index than thecores, the cores being disposed parallel to the first beam direction; anupper clad layer having a generally lower refractive index than thecores (also referred to as the bottom buffer); a priming layer; aplaten; and further comprising: means for coupling light from theillumination means into the first TIR light guide; means for couplinglight out of the waveguide into an output optical path; and a detectorcomprising at least one photosensitive element, wherein ITO electrodesare applied to the opposing faces of the substrate and the waveguidecore;

At step 502 an external material contacting a point on the externalsurface of the platen;

At step 502 sequentially switching elements of the first SBG array intoa diffracting state, all other elements being in their non-diffractingstates;

At step 503 sequentially switching elements of the second SBG array intoa diffracting state, all other elements being in their non-diffractingstates;

At step 504 each diffracting SBG element of the first SBG arraydiffracting incident first TIR light upwards into a first optical path,

At step 505 the transmission grating diffracting the first optical pathlight upwards into a second optical path,

At step 506 a portion of the second optical path light incident at thepoint on the platen being transmitted out of the platen and lightincident on the outer surface of the platen in the absence of saidcontact with an external material being reflected downwards in a thirdoptical path, said third optical path traversing said cores,

At step 508 an active SBG element of the second SBG array along thethird optical path diffracting the third angle light downwards into afourth optical path,

At step 508 the fourth optical path light being reflected upwards into afifth optical path at the third substrate, the fifth optical path lightexceeding the critical angle set by the core/clad interface and thecritical angle set by one of the core/second SBG array or second SBGarray/third substrate interfaces, and proceeding along a TIR path to thedetector.

In one embodiment of the invention the first to fifth optical paths inthe method of FIG. 14 lie in a plane orthogonal to the first SBG array.

In one embodiment of the invention the method of FIG. 14 furthercomprises the step of providing a transparent slab of index lower thanthe third transparent substrate disposed between the third substrate andthe transmission grating, such that the fourth optical path light isreflected upwards at the substrate into a fifth optical path and thefifth optical path light exceeds the critical angle set by the core/cladinterface and the critical angle set by one of the core/second SBGarray, second SBG array/third substrate or third substrate/slabinterfaces, providing a TIR path to the detector.

A contact image sensor according to the principles of the invention isillustrated in the schematic side elevation view of FIG. 15. Theapparatus is identical to that of FIG. 1 but further comprises a halfwave retarder array 3 disposed between the air gap 15 and thetransmission grating 43. The half wave retarder array 3 comprises anarray of column-shaped elements 30 sandwiched by transparent substrates31,32. Each retarder element in the half wave retarder array isswitchable between a polarization rotating state in which it rotates thepolarization of incident light through ninety degrees and a nonpolarization rotating state.

The column elements of the half wave retarder array have longerdimensions disposed parallel to the Y-axis i.e. orthogonally to thefirst TIR beam direction. Each half wave retarder array element overlapsat least one strip element of the first SBG array. At any time oneelement of the first SBG array is in a diffracting state and isoverlapped by an element of the half wave retarder array in itsnon-polarization rotating state, one element of the second SBG array isin a diffracting state, all other elements of the first and second SBGarrays are in a non-diffracting state and all other elements of the halfwave retarder array are in their polarization rotating states.

Turning now to FIG. 16 the function of the half wave retarder array isto control stray light such as that indicated by the ray 220 which isdiffracted by the residual refractive index modulation of the element24. The switchable half wave retarder array solves the problem ofbackground leakage noise by converting unwanted light at source intoS-polarized light. The active (i.e. diffracting) SBG column element 23diffracts light 204 out of the light guide through the element 33 of thehalf wave retarder array 30 array as light 205. Since the element 33 isin its non-polarization rotating state the light 205 remainsP-polarized. Note that all other elements of the half wave retarderarray are in their polarization rotating states. The diffracted ray 220is transmitted through the half wave retarder element 34 which is in itspolarization rotating state such that the P-polarized light 220 isconverted into S-polarized light 221. The ray 221 is next diffractedinto the ray 222 by the transmission grating 43. The ray 223 isreflected off the platen/air interface into a downwards path as the ray223. Since the ray 223 is S-polarized it is not diffracted by the secondSBG and is therefore not coupled into the waveguide path to thedetector. In one embodiment of the invention the light 223 propagatesdownwards though the stack of optical layers until it emerges from thebottom of the illuminator means 1 and is absorbed by a light-trappingmeans which is not illustrated. Typically, the light-trapping meanswould be an absorber. Other means for disposing of light of the typerepresented by the ray 223 will be apparent to those skilled in the artof optical design. The invention does not assume any particular meansfor disposing of such stray light.

In one embodiment of the invention illustrated in FIG. 17 there isprovided a means for contact imaging of an object that emits light of asecond wavelength when illuminated by light of a first wavelength. Theapparatus of FIG. 17 is identical to the sensor FIG. 4 except that inFIG. 15 the rays 237,238,239 which replace the ray 207,208,209 of FIG. 4now correspond to second wavelength light emitted from the object 63which is in contact with the platen and illuminated by first wavelengthlight 206. In one embodiment of the invention the object 63 may be afluorescent material excited by UV radiation. The ray 243 which replacesthe ray 223 of FIG. 4 again represents a stray light path. It should benoted that the embodiment of FIG. 17 will required a more intense lightsource to compensate for the low coupling efficiency of the secondwavelength light into the detector waveguide. The reason for this isthat the light emitted from the object 61 will tend to be diffuse andunpolarized (in contrast to the situation in FIG. 4 where the downgradelight from the platen will be collimated and will retain the incidentlight polarization and collimation).

FIG. 18 is a table of the optical prescriptions of each layer(refractive index at 785 nm. and layer thickness in microns) of atypical implementation of the embodiment of FIG. 1. Each layer isreference by the numerals used in FIG. 1. As should be apparent to thoseskilled in the art many other combinations of layer materials andthickness may be used.

In the above described embodiments the contact sensor essentiallycomprises three modules: a scanner a detector and the platen. Thesecomponents are illustrated in FIGS. 19-20 in which FIG. 19 isessentially FIG. 1 with the components comprising the detector layer 9(that is the second SBG array 4 and the waveguiding 5) contained in adashed line box. The platen comprises the illuminator module and thefirst SBG array device 2. We now consider an embodiment of the inventionthat combines the functions of the second SBG array 4 and thewaveguiding 5 (that is, the detector module as defined above) in asingle SBG array device. This alternative detector module is nowdiscussed with reference to FIGS. 21-26.

As already discussed the contact sensor comprises the following paralleloptical layers configured as a stack: an illumination means forproviding a collimated beam of first polarization light; a first SBGarray device further comprising first and second transparent substratessandwiching an array of selectively switchable SBG column, andtransparent electrodes applied to opposing faces of the SBG substratestogether providing a first TIR light guide for transmitting light in afirst TIR beam direction; and a transmission grating ; and a platen, asillustrated in FIG. 1 but not shown in FIGS. 21-26.

The second SBG array device further comprising third and fourthtransparent substrates 46A,46B sandwiching the SBG layer which isgenerally indicated by 48 and will be explained in more detail next. Thelayer essentially consists of a multiplicity of high index HPDLC regionsseparated by low index HPDLC regions. Patterned transparent electrodes47A,47B are applied to opposing faces of the substrates. The high indexregions provide waveguiding cores disposed parallel to the first beamdirection generally indicated by 250. The low index HPDLC regionsprovide waveguide cladding. The waveguide structure is shown in planview in FIG. 23 which shows a waveguide core 77 and adjoining claddingregions 77A,77B. The waveguide structure is shown in cross section inFIG. 24 which also shows electrodes 47A, 47B across a cladding region77A. The adjacent core region is indicated by 77. Anti-phase voltagesV1,V2 are applied to the upper and lower electrodes via connections 53A,53B using the anti-phase voltage generators 54A,54B. The third andfourth substrate layers 46A, 46B have a generally lower refractive indexthan the cores and will typically match the indices of the claddingregions The patterned electrodes applied to the third substrate comprisecolumn shaped elements such as 55 defining a multiplicity of selectivelyswitchable columns of SBG elements such as the one indicted by 26 whichare aligned orthogonally to the waveguiding cores shown in FIG. 26. Thepatterned electrodes applied to the fourth substrate comprise elongateelements such as 56 overlapping the low index HPDLC regions.

As in the embodiment of FIG. 1 the apparatus further comprises: meansfor coupling light from the illumination means into the first TIR lightguide; means for coupling light out of the second SBG array device intoan output optical path; and a detector 80 comprising at least onephotosensitive element 89 in FIG. 23. The detector comprises an array ofphotosensitive elements, each photosensitive element being opticallycoupled to at least one waveguiding core. Each SBG element in the firstand second SBG arrays is switchable between a diffracting state and anon-diffracting state with the SBG elements diffracting only firstpolarization light.

In one embodiment of the invention based on an SBG waveguiding structurethe SBGs operate in reverse mode such that the diffracting state existswhen an electric field is applied across the SBG element and a nondiffracting state exists when no electric field is applied.Alternatively the SBGs may operate in forward mode, that is thediffracting state exists when no electric field is applied across theSBG element and a non diffracting states exists when an electric fieldis applied. At any time one element of the first SBG array is in adiffracting state, one element of the second SBG array is in adiffracting state, all other elements of the first and second are in anon-diffracting state. An air gap may be provided between first SBGarray and the transmission grating. Alternatively a low refractive indexmaterial may be used for this purpose.

In one embodiment based on an SBG waveguiding structure discussed abovean active SBG element of the first SBG array in a diffracting statediffracts incident first TIR light upwards into a first beam direction.Referring to FIG. 22, light incident on the outer surface of the platenin the absence of external material is reflected downwards in a thirdoptical path 275. The third optical path traverses the cores. An activecolumn 49 of the second SBG array along the third beam directiondiffracts the third angle light into a second TIR path 276 down thetraversed core towards the detector. The first to third optical pathsand the first and second TIR paths are in a common plane.

In one embodiment based on an SBG waveguiding structure the output fromdetector array element is read out in synchronism with the switching ofthe elements of the first SBG array.

In one embodiment based on an SBG waveguiding structure there isprovided a method of making a contact image measurement comprising thesteps of:

-   i) providing an apparatus comprising the following parallel optical    layers configured as a stack: an illumination means for providing a    collimated beam of first polarization light; a first SBG array    device further comprising first and second transparent substrates    sandwiching an array of selectively switchable SBG column elements,    and transparent electrodes applied to opposing faces of the    substrates and the SBG substrates together providing a first TIR    light guide for transmitting light in a first beam direction; a    transmission grating; a transparent substrate; a second SBG array    device further comprising third and fourth substrates sandwiching a    multiplicity of high index HPDLC regions separated by low index    HPDLC regions and patterned transparent electrodes applied to    opposing faces of the substrates; a platen; and a detector; and    further comprising: means for coupling light from the illumination    means into the first TIR light guide; means for coupling light out    of the second SBG array device into an output optical path; and a    detector comprising at least one photosensitive element; the high    index regions providing waveguiding cores disposed parallel to the    first beam direction and the low index HPDLC regions providing    waveguide cladding; the substrates layers having a generally lower    refractive index than the cores, the patterned electrodes applied to    the third substrate defining a multiplicity of selectively    switchable columns orthogonal to the waveguiding cores and the    patterned electrodes applied to the fourth substrate overlapping the    low index HPDLC regions;-   j) an external material contacting a point on the external surface    of the platen;-   k) sequentially switching elements of the first SBG array into a    diffracting state, all other elements being in their non-diffracting    states;-   l) sequentially switching columns of the second SBG array device    into a diffracting state, all other columns being in their    non-diffracting states;-   m) each diffracting SBG element of the first SBG array diffracting    incident first TIR light upwards into a first optical path,-   n) the transmission grating diffracting the first optical path light    upwards into a second optical path,-   o) a portion of the second optical path light incident at the point    on the platen contacted by the external material being transmitted    out of the platen, while portions of said second optical path light    not incident at the point are reflected downwards in a third optical    path, the third optical path traversing one core,-   p) an active SBG column element of the second SBG array along the    third optical path diffracting the third angle light in a second TIR    path down the traversed core and proceeding along a TIR path along    the core to the detector.

In one embodiment of the invention which is illustrated in the schematicside elevation view of FIG. 27 there is provided a contact image sensorusing a single SBG array layer comprising: an illumination means 97 forproviding a collimated beam of first polarization light; an SBG arraydevice further comprising first and second transparent substrates27A,27B sandwiching an array of selectively switchable SBG columns 27,and transparent electrodes (not shown) applied to opposing faces of thesubstrates, said SBG substrates together providing a first TIR lightguide for transmitting light in a first TIR beam direction; a firsttransmission grating layer 91B overlaying the lower substrate of the SBGarray device; a second transmission grating layer 91A overlaying theupper substrates of the SBG array device; a quarter wavelength retarderlayer 99 overlaying the second transmission grating layer; a platen 6overlaying the quarter wavelength retarder layer; and a polarizationrotating reflecting layer 98 overlaying the first transmission gratinglayer. The apparatus further comprises: means 97 for coupling light fromsaid illumination means into said SBG array device; means 96 forcoupling light out of the second SBG array device into an output opticalpath; and a detector (not illustrated) comprising at least onephotosensitive element. The light path from the illumination means tothe platen via a diffracting SBG column 27C is illustrated by the solidline. The path of the reflected light from the platen to the detectormeans is shown as a dashed line.

We next discuss a further embodiment of the invention directed atfurther simplification of the detector component. The new contact sensorarchitecture which is shown in detail in FIG. 28 retains the keyfunctional elements of scanner detector and platen as already discussedabove. As in the case of the SBG waveguide embodiment discussed aboveaim is to eliminate the design complexity and cost of the polymerwaveguide used in the earlier embodiments. In the embodiment of FIG. 28the waveguides are formed by means of passive surface coatings whichconfine the collimated light reflected from the platen to parallelwaveguide-like paths leading to the detector.

In the embodiment of FIG. 28 a contact image sensor comprises: anillumination means for providing a collimated beam of first polarizationlight; an illuminator waveguide for propagating light in a first TIRpath containing a first array of switchable grating columns; a detectorwaveguide for propagating light in a second TIR path containing a secondarray of switchable grating columns; a beam steering means comprising atleast one grating disposed between the platen and the detectorwaveguide; a first waveguide coupler for coupling light from theillumination means into the illuminator waveguide; a second waveguidecoupler for coupling light out of the detector waveguide into an outputoptical path; a detector comprising at least one photosensitive element;and a platen. Each switchable grating element in the first and secondswitchable grating arrays is switchable between a diffracting state anda non-diffracting state. The switchable grating elements diffract onlythe first polarization light. Each external surface of the detectorwaveguide is divided into a first grid of strips interspersed with asecond grid of strips. The first and second grids have differentlight-modifying characteristics. Overlapping strips from the first gridof strips on each external surface are operative to waveguide light.Overlapping strips from the second grid of strips on each externalsurface are operative to absorb light scattered out of regions of thedetector waveguide sandwiched by overlapping strips from the first gridof strips on each external surface. The strips are orthogonal to theswitchable grating columns. The first grid of each external waveguidesurface is one of clear or scattering and the second grid of at leastone external waveguide surface is infrared absorbing. The beam steeringmeans comprises: a first transmission grating layer; a half wavelengthretarder layer overlaying the first transmission grating layer; a secondtransmission grating layer overlaying the half wavelength retarderlayer; and a quarter wavelength retarder layer sandwiched by the secondtransmission grating layer and the platen.

Turning again to FIG. 28 we see that, as in the earlier embodiments, thescanner is an active SBG column array 20 which directs sheets ofcollimated integrated leaser light upwards the transparent detectorlayer into the plate by switching the column elements in scrollingfashion. Typically, the scanning grating comprises 1600 48.8 micron wideITO electrodes etched onto a glass substrate with a 50.8 micron pitch(that is, 500 electrodes per inch). The laser source 35 emits collimatedlight 280 which is coupled into the scanner waveguide by a gratingcoupler 36 into to the TIR path represented by rays 281-284. When apotential is applied across one of the transparent electrodes such as23, light is diffracted out of the scanner waveguide into a directionsuch as 285 (typically normal the waveguide substrate. When the voltageis removed, diffraction ceases and the light continues to be totallyinternally reflected between the scanner substrates. Note that the TIRlight in the scanner is labelled as a being S-polarized (usingS-polarization-sensitive SBGs). In an alternative embodiment the scannermay use conventional P-polarization-sensitive SBGs with a HWF layerbeing provided adjacent the scanner to rotate the out-coupledP-polarized light to S. As will be explained below S-polarized outputlight is required to avoid interaction with the SBGs in the detectorlayer.

As in the earlier embodiments the first and second TIR paths areparallel to each other the switchable grating columns are preferentiallyorthogonal to the TIR paths. The first and second switchable gratingarrays are switched in cyclic fashion with only one the column elementin each array being in a diffracting state at any time. The illuminatorand detector waveguides each comprise first and second transparentsubstrates sandwiching an array of switchable grating columns, andtransparent electrodes applied to opposing faces of the substrates. Theswitchable grating is one of a forward mode SBG, a reverse mode SBG, ora stack of thin switchable gratings. For conventional forward mode SBGsthe diffracting state exists when no electric field is applied acrossthe switchable grating element and the non diffracting states existswhen an electric field is applied. This situation is reversed forreverse mode SBGs. At any time one element of the first switchablegrating array is in a diffracting state, one element of the secondswitchable grating array is in a diffracting state, all other elementsof the first and second switchable grating arrays are in anon-diffracting state. The output from detector array element is readout in synchronism with the switching of the elements of the firstswitchable grating array.

As in the earlier embodiments, the scanned light needs to be directedonto the platen 6 at a preferred angle. This ensures a clear imagecapture that is tolerant to the enrollee's hand and finger moisture.This is accomplished by passive tilt gratings 64B,64D one (64B) for theupward beam 286 and a reversed version (64D) for the downward reflectedlight. The tilt gratings are essentially passive transmission gratingrecorded in holographic polymer film such as the material manufacturedby Bayer Inc. A Quarter Wave Film (QWF) 64A which is sandwiched by theupward beam tilt grating and platen converts the upward goingS-polarized light 287 into circularly polarized light 287A. Onreflection from the platen the sense of the circular polarized light isreversed as indicated by the symbol 287B so that P polarized light 288is produced after the second pass through the QWF. The tilt grating 64Ddiffracts this light normal to the detector layer in the direction 289.

During a scan, the user's four fingers are placed onto the platensurface. Wherever the skin touches the platen, it “frustrates” thereflection process, causing light to leak out of the platen. Thus, theparts of the skin that touch the platen surface reflect very littlelight, forming dark pixels in the fingerprint image. The image is builtup line by line into a 500 dpi, FBI-approved industry standard pictureready for comparison checking.

The detector 65 comprises an SBG column array 65A similar to the scannerarray sandwiched by substrates 65B, 65C. Electrodes (not illustrated)are applied to the opposing surfaces of the substrate with at least onebeing pattern with ITO columns overlaying the SBG column elements. Anoutcoupling grating 38 (or other equivalent optical means such as prism)out couples light 292 from the detector waveguide towards a detectorarray 37. The TIR path in the waveguide from an active SBG columnelement 67 to the outcoupling grating 38 is represented by 290.

The detector and scanner waveguides may be air separated. Alternativethey may be sandwich a low index material layer which is schematicallyindicated by the thin layer 68. Since the scanner waveguide istransparent the out coupled light from the detector waveguide may in analternative embodiment be transmitted through the detector layer onto adetector array which is disposed alongside the laser source. Otherimplementations that will result in further compression of the sensorform factor should be apparent to those skilled in the art.

As shown in FIG. 28 the illumination traverses the detector waveguide onits way to the platen. In another embodiment of the invention theilluminator waveguide may be disposed between the detector waveguide andthe platen (and beam steering gratings) such that light reflection fromthe platen traverses the illuminator waveguide on its way to thedetector waveguide. As illustrated in FIG. 28 the ray directions fromthe source to the detector lie in a common plane.

Each external surface of the detector waveguide is divided into a firstgrid of strips interspersed with a second grid of strips. The first andsecond grids have different light-modifying characteristics. Overlappingstrips from the first grid of strips on each external surface areoperative to confine light to a waveguide path. Overlapping strips fromthe second grid of strips on each external surface are operative toabsorb light scattered out of regions of the detector waveguidesandwiched by overlapping strips from the first grid of strips on eachexternal surface. The strips are orthogonal to the switchable gratingcolumns. The first grid of each external waveguide surface is one ofclear or scattering and the second grid of at least one externalwaveguide surface is infrared absorbing. Essentially, three types ofsurface strip are required: clear, scattering and light (infrared)absorbing. Typically the scattering properties will be provided byfrosting the surface or applying some computer generated surface reliefstructure using an etching process. Other methods of providingcontrolled scatter using diffractive surface structures may also beused. The stripes define parallel propagation channels terminating atthe linear detector array. Typically, the channel widths are 40 micronwith gaps of 12 micron give a pitch of 52 micron equivalent to 500 dpi.

FIG. 28B and FIG. 28C are plan views of the bottom and top of thedetector. The bottom surface 69A of the detector (that is, the onenearest the scanner) has alternating clear regions such as 39A andregions to which a frost etch has been applied such as 39B. The topsurface 9B of the detector (that is, the one nearest the platen hasalternating clear regions such as 39C and regions to which an infraredabsorbing thin film has been applied. The infrared absorbing coatingregions of the top surface overlay the clear regions of the bottomsurface. FIG. 28D is a cross section of the detector waveguide showinglight spots emerging from the waveguiding structure of FIGS. 28B-28C. Across section of the detector waveguide showing the SBG array,substrates and upper and lower surface coatings are provided in FIGS.29-30. As indicated in FIG. 30 the SBG array comprises the columnelements 66A separated by small gaps 66D. The external faces of thedetector waveguide and the illuminator waveguide abut an air space or alow refractive index material layer.

Collimated reflected beams from the platen enter the detector layer inthe gaps between the IR absorbing stripes and undergo TIR within up tothe detector array as indicated by the rays 293,294. Hence the beampropagation is analogous to that provided by waveguide cavities. Sincethere will be collimation errors owing to imperfections in the laserscollimation optics and a small amount of scatter from the PDLC materialand optical interfaces, there is a risk of cross talk between adjacentdetector channels as indicted by the ray 295. The combination of the IRabsorbing layers and frosted surfaces overcome this problem. Lightscattered out of a give channel is scattered by the frosted layer andabsorbed by the IR coating. Any forward scattered light or multiplescatter between near neighboring channels will tend to diminish inintensity with each ray surface interaction and will form a backgroundnoise level that can be subtracted from the fingerprint signature by theprocessing software. In one embodiments shown in FIG. 31 a variation onthe above detector design uses alternating clear regions of IR absorberstripes at the top (39A,39B) and bottom (39F,39G) of the waveguideinstead of the IR absorber/frost etch arrangement of FIGS. 28B-28C.

As shown in FIG. 32 many different combinations of strips may be used.The strip combinations are illustrated schematically by the 5×2 matriceslabelled 38H-39N in which the top row represents the strips applied tothe upper surface of the detector waveguide and the bottom rowrepresents the strips applied to the bottom surface of the detectorwaveguide. The light-modifying strips are labelled by characters A(absorbing), F (frosted). The matrix cells containing no charactersindicate clear strips.

It should be noted that in most implementations whether a particularstrip pattern is at the top or bottom of the waveguide is not critical.It is of course necessary to ensure that at least one of the strips onthe waveguide surface nearest the platen is transparent to allow lightreflected from the platen to enter the detector waveguide.

In one embodiment the scanner SBG operates in reverse mode. That is theSBG columns diffract only when an electric field is applied across theITO electrodes. With normal mode SBGs the noise from diffraction andscatter occurring within the gaps between the electrodes would swamp theoptical signal.

The linear array of photo detectors 37B, is connected to the detectorlayer via an array of micro lenses 37A as shown in the schematicillustration of FIG. 33. Alternatively the detector may marry updirectly to the frosted surface of the detector layer as shown in FIG.34. The illumination of the platen outer surface by a light sheet 300containing the incident ray 301 which is reflected into the ray path 302and is coupled into a TIR path 303 incised a waveguide region 39A of thedetector waveguide 65 is shown. The TIR light is coupled via a micronsarray 37A into an element of the detector array 37B by means of anoutcoupling grating 38.

The linear detector may be based on any fast, high resolution arraytechnology. One candidate technology would be CCD. An alternativetechnology that may be used is the Contact Image Sensors (CIS) which israpidly replacing CCDs in low cost low power and portable applicationssuch as copiers, flatbed scanners as well as barcode readers and opticalidentification technology. A typical CIS will provide high speedsensing; high speed ADC 12 bit 600 dpi. At the time of writing anexemplary CIS is Mitsubishi Electric WC6R305X. Current CIS will not haveas high sensitivity as the best commercially available CCD arrays. Withcollimated laser illumination a CIS detector can be highly powerefficient, allowing scanners to be powered through the minimal linevoltage supplied via a USB connection. From the ergonomic perspective, aCIS contact sensor is smaller and lighter than a CCD line sensor, andallows all the necessary optical elements to be included in a compactmodule, thus helping to simplify the inner structure of the scanner. TheCIS greatly simplifies the sensor electronics. Many other detectorconfigurations may be used with the invention. In one embodiment twolinear arrays may be combined. However, such embodiments requirecomplicated waveguiding and electronics routing and output signalstitching.

In one embodiment a method of making a contact image measurement usingthe apparatus of FIG. 28 is provided comprising the steps of:

-   a) providing an apparatus comprising: an illumination means for    providing a collimated beam of first polarization light; an    illuminator waveguide for propagating light in a first TIR beam    direction containing a first array of switchable grating columns; a    detector waveguide for propagating light in a first TIR beam    direction containing a second array of switchable grating columns; a    beam steering means comprising at least one grating disposed between    the platen and the detector waveguide; a first waveguide coupler for    coupling light from the illumination means into the illuminator    waveguide; a platen; a second waveguide coupler for coupling light    out of the detector waveguide into an output optical path; and a    detector comprising at least one photosensitive element. The    external surfaces of the detector waveguide comprise interspersed    multiplicities of strips with different light modifying    characteristics. The strips are orthogonal to the switchable grating    columns, each light modifying strip overlapping a clear strip;-   b) coupling light from the illumination means into the illuminator    waveguide;-   c) an external material contacting a point on the external surface    of the platen;-   d) sequentially switching elements of the first switchable grating    array into a diffracting state, all other elements being in their    non-diffracting states;-   e) sequentially switching columns of the second switchable grating    array into a diffracting state, all other columns being in their    non-diffracting states;-   f) each diffracting switchable grating element of the first    switchable grating array diffracting incident first TIR light    upwards into a first optical path;-   g) the beam steering means deflecting the first optical path light    into a second optical path;-   h) a portion of the second optical path light incident at the point    on the platen contacted by the external material being transmitted    out of the platen, portions of the second optical path light not    incident at the point being reflected into a third optical path;-   i) an active switchable grating column element of the second    switchable grating array along the third optical path diffracting    the third angle light in a second TIR path; and-   j) coupling light out of the detector waveguide towards the    detector.

A method of making a contact image measurement in one embodiment of theinvention (using the apparatus of FIG. 28) in accordance with the basicprinciples of the invention is shown in the flow diagram in FIG. 35.Referring to the flow diagram, we see that the said method comprises thefollowing steps.

At step 550 provide a light source; a platen; an illuminator waveguidecontaining a first array of SBG elements; a detector waveguidecontaining a second array of SBG elements, external surfaces of thedetector waveguide being divided into interspersed grids oflight-modifying strips, a beam steering grating system; a first couplerfor coupling light into the illuminator waveguide; a second coupler forcoupling light out of the detector waveguide towards a detector.

At step 551 couple light from light source into TIR path in illuminatorwaveguide.

At step 552 an external material of lower refractive index than saidplaten contacts a point on the external surface of said platen.

At step 553 sequentially switch first SBG array elements intodiffracting state.

At step 554 sequentially switch elements of second SBG array into adiffracting state, all other elements being in non-diffracting states.

At step 555 each diffracting SBG element of first SBG array diffractsincident light into a first optical path.

At step 556 beam steering grating system diffracts first optical pathlight into second optical path.

At step 557 second optical path light incident at said point on platenis reflected in a third optical path.

At step 558 active SBG elements of second SBG array along third opticalpath diffract third angle light into TIR path in detector waveguide.

At step 558 couple light out of detector waveguide towards detector.

FIG. 36 shows a further embodiment of the invention that combinespolymer waveguides of the type discussed earlier with the lightmodifying stripe principle used in the embodiment of FIG. 28. Since thebasic principles of the ray propagation and diffraction by elements ofthe SBG arrays have already been discussed in some detail in relation tothe embodiment of FIG. 28 only an outline description is provided here.The platen, beam steering grating layers and illuminator waveguide areidentical to the ones illustrated in FIG. 28. However, the detectorwaveguide comprises a polymer waveguide layer 69C onto which is overlaida SBG array 65 comprising the SBG array 65A sandwiched by the substrates65B,65C to which electrodes are applied as discussed above. The uppersurface of the substrate 65B which is labelled 69D in the plan view ofFIG. 36D has infrared absorbing stripes 39A interspersed with clearstripes 39B. The waveguide layer 70 comprises waveguide cores 71 incladding material 72. The design issues relating to the design ofwaveguides for use with the invention have been discussed in detailearlier (see FIGS. 4-11 and the accompanying description). The ray pathfrom the source to the detector 37 is indicated by the rays 280-312.

Although the switchable grating arrays used in the detector andilluminator waveguide components essentially one dimension arrays ofcolumn shaped elements (as shown in FIG. 3) it is possible to apply theinvention using two dimensional arrays. FIG. 37 shows two operationalstates of a two dimension switchable grating array 20C (for use in oneor both of the detector and illumination SBG arrays) containingaddressable pixels 20D. In FIG. 37A a pixel 20E is in a diffractingstate while in FIG. 37B a neighboring pixel 20F is in a diffractingstate. The pixels could be switched one column at a time. However thereal benefit of two dimensional arrays lies in enabling moresophisticated image sensing strategies, for example area-of-interest-based image acquisition. Such applications of the inventionwill require fast detectors and fast switching of the grating arrays.

FIG. 38 shows a preliminary software architecture for use in afingerprint scanning implementation of the invention. In a typicalmobile application of the invention the preferred software platformwould be a ruggedized computer tablet such as, for example, thePanasonic Android Touchpad. It is expected that Microsoft Windows 8computer tablet technology will stimulate further product development inthese areas. Desirably, any platform should provide an integrated GPSmodule. FIG. 38 illustrates one possible implementation. The systemcomponents implemented on the software platform 520 comprise anexecutive program 521, biometric software 522, hardware control 523,finger print server 524, fingerprint database 525, graphical userinterface (GUI)526 and communication interfaces 527. The biometricsoftware will typically provide 1:1 and 1:N comparisons; noise removal,matching algorithms, image enhancement and options for saving images.The hardware control module includes software for control theelectronics for detector channel switching and readout, illuminatorcomponent switching, laser control and basic functions such as an on/offswitch. Communication interfaces will typically include LAN, WAN andINTERNET. FIG. 38 also shows the biometric scanner 530 comprising 512element detector array, SBG array driver 532, detector 533, illuminatorcomponent 534 and laser module 535. System Development Kits (SDKs) forimplementing the functionalities shown in FIG. 39 are currentlyavailable. They can be categorized into low and high level tools. Whilelow level tools can provide rapid integration they still require thedevelopment of a robust fingerprint reader software matching server andother vital elements for dealing with problems such as exceptionhandling and system optimization, which makes embedding them intoapplications problematic. When modifications or enhancements are made toeither the host application or to the fingerprint SDK the host softwaremust be recompiled with the fingerprint SDK, leading to ongoing supportand maintenance problems. High level SDKs free the user from needing tounderstand the parameters involved with fingerprint comparison, how theywork, why they are significant, and how data needs to be extracted froman image as well as data type mapping, database management, datasynchronization, exception handling. The ability to perform 1:Ncomparison for large databases is a highly desirable feature importantfeature; opening a record set from the database and matching one-by-onewill not produce fast results. In general high level SDKs will be betterat handling poor image quality, bad image acquisition, and unpredictableuser input. Desirably the SDK should support a variety of developmentenvironments including: C++, VB, NET, Delphi, PowerBuilder, Java,Clarion, and web applications. High level SDKs avoid the need fordevelopment of special DLLs which can consume 6-12 months indevelopment.

In applications such as finger print sensing the illumination light isadvantageously in the infrared. In one embodiment of the invention thelaser emits light of wavelength 785 nm. However, the invention is notlimited to any particular illumination wavelength.

In fingerprint detection applications the invention may be used toperform any type “live scan” or more precisely any scan of any printridge pattern made by a print scanner. A live scan can include, but isnot limited to, a scan of a finger, a finger roll, a flat finger, a slapprint of four fingers, a thumb print, a palm print, or a combination offingers, such as, sets of fingers and/or thumbs from one or more handsor one or more palms disposed on a platen. In a live scan, for example,one or more fingers or palms from either a left hand or a right hand orboth hands are placed on a platen of a scanner. Different types of printimages are detected depending upon a particular application. A flatprint consists of a fingerprint image of a digit (finger or thumb)pressed flat against the platen. A roll print consists of an image of adigit (finger or thumb) made while the digit (finger or thumb) is rolledfrom one side of the digit to another side of the digit over the surfaceof the platen. A slap print consists of an image of four flat fingerspressed flat against the platen. A palm print involves pressing all orpart of a palm upon the platen.

The present invention essentially provides a solid state analogue of amechanical scanner. The invention may be used in a portable fingerprintsystem which has the capability for the wireless transmission offingerprint images captured in the field to a central facility foridentity verification using an automated fingerprint identificationsystem.

It should be emphasized that the drawings are exemplary and that thedimensions have been exaggerated.

It should be understood by those skilled in the art that while thepresent invention has been described with reference to exemplaryembodiments, it is to be understood that the invention is not limited tothe disclosed exemplary embodiments. Various modifications,combinations, sub-combinations and alterations may occur depending ondesign requirements and other factors insofar as they are within thescope of the appended claims or the equivalents thereof.

What is claimed is:
 1. A waveguide device comprising: an opticalsubstrate with first and second light reflecting surfaces; and a lightabsorbing coating applied to at least one of said surfaces with portionsof the coating removed to provide at least one non-absorbing regiondisposed between light absorbing regions, each said non-absorbing regionof said first surface overlapping a non-absorbing region of said secondsurface to form a waveguiding volume within said substrate.
 2. Theapparatus of claim 1, wherein said non-absorbing regions are portions ofsaid substrate surface at which total internal refection can take place.3. The apparatus of claim 1, wherein said non-absorbing regions areportions of said light reflecting surface textured to scatter light orcoated to provide partial reflection of light.
 4. The apparatus of claim1, wherein said waveguide volume provides a rectangular waveguidingcore.
 5. The apparatus of claim 1, further comprising a source of lightoptically coupled to said waveguide.
 6. The apparatus of claim 5,wherein said light is collimated before being coupled into saidwaveguide.
 7. The apparatus of claim 5, wherein said light is infraredor ultraviolet.
 8. The apparatus of claim 5, wherein said light ispolarized before being coupled into said waveguide.
 9. The apparatus ofclaim 5, further comprising at least one of a polarization selectionlayer, a polarization rotation layer, a platen for contact imageformation, a transparent substrate, a layer of air, a layer of low indexmaterial, or a grating layer disposed between said source and saidwaveguide.
 10. The apparatus of claim 1, further comprising at least onegrating for coupling light into said waveguiding volume.
 11. Theapparatus of claim 10, wherein said grating is one of a forward modeswitchable Bragg grating, a reverse mode switchable Bragg grating, astack of thin switchable gratings or a surface relief grating.
 12. Theapparatus of claim 10, wherein said grating at least partially overlapssaid waveguiding volume.
 13. The apparatus of claim 10, wherein saidgrating is patterned into a multiplicity of elongate grating elementsaligned orthogonal to a light propagation direction of said waveguidingvolume.
 14. The apparatus of claim 10, wherein said optical substrate isdivided into a pair of substrates sandwiching said grating, andtransparent electrodes are applied to a surface of each substrate. 15.The apparatus of claim 10, wherein said grating comprises a plurality ofgrating elements switchable between a diffracting state and anon-diffracting state, wherein said grating elements are switchedsequentially.
 16. The apparatus of claim 10, wherein said grating is areflection grating.
 17. The apparatus of claim 1, further comprising adetector optically coupled to said waveguide.
 18. The apparatus of claim1, wherein said light absorbing regions and said non-absorbing regionsform a grid pattern.
 19. The apparatus of claim 1, wherein externalfaces of said waveguide abut air or a low refractive index materiallayer.
 20. The apparatus of claim 1 wherein said waveguide provides adetector waveguide in a contact image sensor comprising: an illuminationsource providing a collimated beam of light; an illuminator waveguideconfigured for propagating said light in a first total internalreflection path and containing a first array of grating columns; adetector comprising at least one photosensitive element; a platen; saiddetector waveguide configured for propagating light in a second totalinternal reflection path and containing a second array of gratingcolumns; at least one beam steering grating disposed between said platenand said waveguide; a first waveguide coupler for coupling light fromsaid illumination source into said illuminator waveguide; and, a secondwaveguide coupler for coupling light out of said detector waveguide intoan output optical path, wherein said grating elements of said firstarray are orthogonal to said first total internal reflection path andsaid grating elements of said second array are orthogonal to said secondtotal internal reflection path, wherein said columns in said first andsecond grating arrays are switched in cyclic fashion with only one saidcolumn element in each array being in a diffracting state at any time.